Add general Xamarin sample for vision models. (#58)

* Add general Xamarin sample for vision modles.
Currently demonstrates image classification and face recognition.
More models can be added in the future for SSD etc.

mobile/examples/Xamarin/.gitignore

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+/Models

mobile/examples/Xamarin/VisionSample/VisionSample.Forms.Android/Assets/AboutAssets.txt

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+Any raw assets you want to be deployed with your application can be placed in
+this directory (and child directories) and given a Build Action of "AndroidAsset".
+
+These files will be deployed with your package and will be accessible using Android's
+AssetManager, like this:
+
+public class ReadAsset : Activity
+{
+    protected override void OnCreate (Bundle bundle)
+    {
+        base.OnCreate (bundle);
+
+        InputStream input = Assets.Open ("my_asset.txt");
+    }
+}
+
+Additionally, some Android functions will automatically load asset files:
+
+Typeface tf = Typeface.CreateFromAsset (Context.Assets, "fonts/samplefont.ttf");

mobile/examples/Xamarin/VisionSample/VisionSample.Forms.Android/MainActivity.cs

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+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Android.App;
+using Android.Content.PM;
+using Android.Runtime;
+using Android.OS;
+using Microsoft.ML.OnnxRuntime;
+
+namespace VisionSample.Forms.Droid
+{
+    [Activity(Label = "VisionSample.Forms", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true,
+              ScreenOrientation = ScreenOrientation.Portrait,
+              ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode |
+                                     ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize)]
+    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
+    {
+        protected override void OnCreate(Bundle savedInstanceState)
+        {
+            base.OnCreate(savedInstanceState);
+
+            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
+            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
+
+            SessionOptionsContainer.Register(
+                nameof(ExecutionProviderOptions.Platform),
+                (sessionOptions) => sessionOptions.AppendExecutionProvider_Nnapi(NnapiFlags.NNAPI_FLAG_USE_NONE));
+
+            LoadApplication(new App());
+        }
+
+        public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Permission[] grantResults)
+        {
+            Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
+
+            base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
+        }
+    }
+}

mobile/examples/Xamarin/VisionSample/VisionSample.Forms.Android/Properties/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.microsoft.onnxruntime_visionsample_forms">
+	<uses-sdk android:minSdkVersion="27" android:targetSdkVersion="30" />
+	<application android:label="VisionSample.Forms.Android" android:theme="@style/MainTheme">
+		<provider android:name="androidx.core.content.FileProvider" android:authorities="${applicationId}.fileprovider" android:exported="false" android:grantUriPermissions="true">
+			<meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/file_paths"></meta-data>
+		</provider>
+	</application>
+	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+</manifest>

mobile/examples/Xamarin/VisionSample/VisionSample.Forms.Android/Properties/AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.InteropServices;
+using Android.App;
+
+// General Information about an assembly is controlled through the following 
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("VisionSample.Forms.Android")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("VisionSample.Forms.Android")]
+[assembly: AssemblyCopyright("Copyright ©  2021")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+[assembly: ComVisible(false)]
+
+// Version information for an assembly consists of the following four values:
+//
+//      Major Version
+//      Minor Version 
+//      Build Number
+//      Revision
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]
+
+// Add some common permissions, these can be removed if not needed
+[assembly: UsesPermission(Android.Manifest.Permission.Internet)]
+[assembly: UsesPermission(Android.Manifest.Permission.WriteExternalStorage)]
+
+// Needed for Picking photo/video
+[assembly: UsesPermission(Android.Manifest.Permission.ReadExternalStorage)]
+
+// Needed for Taking photo/video
+[assembly: UsesPermission(Android.Manifest.Permission.Camera)]
+
+// Add these properties if you would like to filter out devices that do not have cameras, or set to false to make them optional
+[assembly: UsesFeature("android.hardware.camera", Required = true)]
+[assembly: UsesFeature("android.hardware.camera.autofocus", Required = true)]

mobile/examples/Xamarin/VisionSample/VisionSample.Forms.Android/Resources/AboutResources.txt

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+Images, layout descriptions, binary blobs and string dictionaries can be included 
+in your application as resource files.  Various Android APIs are designed to 
+operate on the resource IDs instead of dealing with images, strings or binary blobs 
+directly.
+
+For example, a sample Android app that contains a user interface layout (main.xml),
+an internationalization string table (strings.xml) and some icons (drawable-XXX/icon.png) 
+would keep its resources in the "Resources" directory of the application:
+
+Resources/
+    drawable-hdpi/
+        icon.png
+
+    drawable-ldpi/
+        icon.png
+
+    drawable-mdpi/
+        icon.png
+
+    layout/
+        main.xml
+
+    values/
+        strings.xml
+
+In order to get the build system to recognize Android resources, set the build action to
+"AndroidResource".  The native Android APIs do not operate directly with filenames, but 
+instead operate on resource IDs.  When you compile an Android application that uses resources, 
+the build system will package the resources for distribution and generate a class called
+"Resource" that contains the tokens for each one of the resources included. For example, 
+for the above Resources layout, this is what the Resource class would expose:
+
+public class Resource {
+    public class drawable {
+        public const int icon = 0x123;
+    }
+
+    public class layout {
+        public const int main = 0x456;
+    }
+
+    public class strings {
+        public const int first_string = 0xabc;
+        public const int second_string = 0xbcd;
+    }
+}
+
+You would then use R.drawable.icon to reference the drawable/icon.png file, or Resource.layout.main 
+to reference the layout/main.xml file, or Resource.strings.first_string to reference the first 
+string in the dictionary file values/strings.xml.