[Mono.Android] Remove more LINQ usage (#5443)

Context: #5442

Apk size of BuildReleaseArm64False/net6 comparison:

    apkdiff --bs -e dll$ before.apk after.apk
    Size difference in bytes ([*1] apk1 only, [*2] apk2 only):
      +         274 assemblies/Mono.Android.dll
        Type Android.Runtime.AndroidTypeManager
          -          42 Method System.Collections.Generic.IEnumerable`1<System.Type> GetTypesForSimpleReference (string)
          -          33 Method public void RegisterNativeMembers (Java.Interop.JniType, System.Type, string)
          +             Method System.Collections.Generic.IEnumerable`1<System.Type> <>n__0 (string)
          -             Type <>c__DisplayClass11_0
          -             Type <>c__DisplayClass11_1
          +             Type <GetTypesForSimpleReference>d__2
      -          67 assemblies/System.Private.CoreLib.dll
        Type System.Array
          -             Method public static void Fill (!!T[], !!T)
      -         424 assemblies/System.Linq.dll
        Type System.Linq.Enumerable
          -             Method public static System.Collections.Generic.IEnumerable`1<!!TResult> Repeat (!!TResult, int)
          -             Type RepeatIterator`1<TResult>
        Type System.Linq.ThrowHelper
          -             Method static void ThrowArgumentOutOfRangeException (System.Linq.ExceptionArgument)
    Summary:
      -         217 Assemblies -0.03% (of 782,054)

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -246,11 +246,12 @@ public AndroidTypeManager (bool jniAddNativeMethodRegistrationAttributePresent)
 
 		protected override IEnumerable<Type> GetTypesForSimpleReference (string jniSimpleReference)
 		{
+			foreach (var ti in base.GetTypesForSimpleReference (jniSimpleReference))
+				yield return ti;
+
 			var t = Java.Interop.TypeManager.GetJavaToManagedType (jniSimpleReference);
-			if (t == null)
-				return base.GetTypesForSimpleReference (jniSimpleReference);
-			return base.GetTypesForSimpleReference (jniSimpleReference)
-				.Concat (Enumerable.Repeat (t, 1));
+			if (t != null)
+				yield return t;
 		}
 
 		protected override string? GetSimpleReference (Type type)
@@ -395,7 +396,13 @@ public override void RegisterNativeMembers (JniType nativeClass, Type type, stri
 				Delegate callback;
 				if (toks [2] == "__export__") {
 					var mname = toks [0].Substring (2);
-					var minfo = type.GetMethods (BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static).Where (m => m.Name == mname && JavaNativeTypeManager.GetJniSignature (m) == toks [1]).FirstOrDefault ();
+					MethodInfo? minfo = null;
+					foreach (var mi in type.GetMethods (BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static))
+						if (mi.Name == mname && JavaNativeTypeManager.GetJniSignature (mi) == toks [1]) {
+							minfo = mi;
+							break;
+						}
+
 					if (minfo == null)
 						throw new InvalidOperationException (String.Format ("Specified managed method '{0}' was not found. Signature: {1}", mname, toks [1]));
 					callback = CreateDynamicCallback (minfo);