[Mono.Android] remove IEnumerable iterators called at startup

When profiling with `adb setprop debug.mono.profile log:alloc`, I
noticed the following allocations:

    Allocation summary
         Bytes      Count  Average Type name
         84840        505      168 Java.Interop.JniRuntime.JniTypeManager.<CreateGetTypeSignaturesEnumerator>d__11
         24096        502       48 Android.Runtime.AndroidTypeManager.<GetSimpleReferences>d__1

This was using the Xamarin.Forms integration project in xamarin-android:

https://github.com/xamarin/xamarin-android/tree/master/tests/Xamarin.Forms-Performance-Integration

100K of memory allocations are a bit much, so seemed like it might help
to reduce these.

I made a version of both `JniTypeManager.CreateTypeSignatures` and
`AndroidTypeManager.GetSimpleReferences` that return a single value
instead of an `IEnumerable` and `yield return`.

Other general performance changes:

* Added `[MethodImpl (MethodImplOptions.AggressiveInlining)]` to
  `JniTypeManager.AssertValid`. It is called quite frequently.

* Removed a call to `.GetTypeInfo()`, as this was not needed. It wraps
  `System.Type` in another type that was probably originally used for
  netstandard 1.x compatibility.

See: https://github.com/mono/mono/blob/c5b88ec4f323f2bdb7c7d0a595ece28dae66579c/mcs/class/referencesource/mscorlib/system/reflection/introspectionextensions.cs#L24

Changes had to be made in both java.interop and xamarin-android for
this to work.

~~ Results ~~

I was able to see a startup improvement in a Release build of the
Xamarin.Forms integration project on a Pixel 3 XL:

    Before:
    01-08 14:53:34.926  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +779ms
    01-08 14:53:38.892  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +764ms
    01-08 14:53:42.859  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +753ms
    01-08 14:53:46.795  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +748ms
    01-08 14:53:50.792  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +764ms
    01-08 14:53:54.759  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +756ms
    01-08 14:53:58.726  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +764ms
    01-08 14:54:02.675  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +749ms
    01-08 14:54:06.691  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +759ms
    01-08 14:54:10.641  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +749ms
    Average(ms): 758.5
    Std Err(ms): 3.0523215208537
    Std Dev(ms): 9.65228815704684
    After:
    01-08 14:55:56.972  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +772ms
    01-08 14:56:00.906  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +741ms
    01-08 14:56:05.006  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +741ms
    01-08 14:56:08.940  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +748ms
    01-08 14:56:12.890  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +747ms
    01-08 14:56:16.839  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +753ms
    01-08 14:56:20.824  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +757ms
    01-08 14:56:24.775  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +746ms
    01-08 14:56:28.776  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +747ms
    01-08 14:56:32.740  1473  1503 I ActivityTaskManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +748ms
    Average(ms): 750
    Std Err(ms): 2.87904305089189
    Std Dev(ms): 9.10433352249844

It seems like this might save ~8ms on startup?

There is also a good improvement to memory usage/allocations.

Looking at Mono's memory summary:

    Before:
        Mono System:
        Working Set                    : 71008256
        Private Bytes                  : 601202688
        Virtual Bytes                  : 1306873856
    After:
        Mono System:
        Working Set                    : 70774784
        Private Bytes                  : 552910848
        Virtual Bytes                  : 1302822912

Then the total GC summary:

    Before:
    Event: GC allocations          : 9340
    After:
    Event: GC allocations          : 8867

Author: jonathanpeppers

external/Java.Interop

@@ -229,11 +229,20 @@ protected override IEnumerable<Type> GetTypesForSimpleReference (string jniSimpl
 				.Concat (Enumerable.Repeat (t, 1));
 		}
 
-		protected override IEnumerable<string> GetSimpleReferences (Type type)
+		protected override string GetSimpleReference (Type type)
 		{
-			foreach (var simpleRef in base.GetSimpleReferences (type)) {
-				yield return simpleRef;
+			var j = JNIEnv.monodroid_typemap_managed_to_java (type.FullName + ", " + type.Assembly.GetName ().Name);
+			if (j != IntPtr.Zero) {
+				return Marshal.PtrToStringAnsi (j);
+			}
+			if (JNIEnv.IsRunningOnDesktop) {
+				return JavaNativeTypeManager.ToJniName (type);
 			}
+			return null;
+		}
+
+		protected override IEnumerable<string> GetSimpleReferences (Type type)
+		{
 			var j = JNIEnv.monodroid_typemap_managed_to_java (type.FullName + ", " + type.Assembly.GetName ().Name);
 			if (j != IntPtr.Zero) {
 				yield return Marshal.PtrToStringAnsi (j);