Get OpenJDK from well-known paths, not just registry (#72)

Context: http://work.azdo.io/735565

Turns Out™ that certain anti-virus programs block or otherwise
mitigate access to various Windows Registry keys.  (No idea which
anti-virus, or what the symptoms are; this is just What I've Heard.)

Reduce reliance on the registry by looking for the JDK within the
`%ProgramW6432%\Android\jdk\microsoft_dist_openjdk_*` directories,
in which the `*` suffix is a `System.Version`-parsable value, and
prefer the highest JDK found there.

The Registry value is still preferred, if set.

nuget.version

@@ -1 +1 @@
-1.0
+1.1

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -134,6 +134,7 @@ IEnumerable<JdkInfo> ToJdkInfos (IEnumerable<string> paths, string locator)
 
 			return ToJdkInfos (GetPreferredJdkPaths (), "Preferred Registry")
 				.Concat (ToJdkInfos (GetOpenJdkPaths (), "OpenJDK"))
+				.Concat (ToJdkInfos (GetKnownOpenJdkPaths (), "Well-known OpenJDK paths"))
 				.Concat (ToJdkInfos (GetOracleJdkPaths (), "Oracle JDK"));
 		}
 
@@ -163,6 +164,36 @@ private static IEnumerable<string> GetOpenJdkPaths ()
 			}
 		}
 
+		/// <summary>
+		/// Locate OpenJDK installations by well known path.
+		/// </summary>
+		/// <returns>List of valid OpenJDK paths in version descending order.</returns>
+		private static IEnumerable<string> GetKnownOpenJdkPaths ()
+		{
+			string JdkFolderNamePattern = "microsoft_dist_openjdk_";
+
+			var paths = new List<Tuple<string, Version>> ();
+			var rootPaths = new List<string> {
+				Path.Combine (Environment.ExpandEnvironmentVariables ("%ProgramW6432%"), "Android", "jdk"),
+				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86), "Android", "jdk"),
+			};
+
+			foreach (var rootPath in rootPaths) {
+				if (Directory.Exists (rootPath))  {
+					foreach (var directoryName in Directory.EnumerateDirectories (rootPath, $"{JdkFolderNamePattern}*").ToList ()) {
+						var versionString = directoryName.Replace ($"{rootPath}\\{JdkFolderNamePattern}", string.Empty);
+						if (Version.TryParse (versionString, out Version ver)) {
+							paths.Add (new Tuple<string, Version>(directoryName, ver));
+						}
+					}
+				}
+			}
+
+			return paths.OrderByDescending (v => v.Item2)
+				.Where (openJdk => ProcessUtils.FindExecutablesInDirectory (Path.Combine (openJdk.Item1, "bin"), _JarSigner).Any ())
+				.Select (openJdk => openJdk.Item1);
+		}
+
 		private static IEnumerable<string> GetOracleJdkPaths ()
 		{ 
 			string subkey = @"SOFTWARE\JavaSoft\Java Development Kit";