[Mono.Android] Make TypeManager a normal Java.Lang.Object (#1537)

Context: https://github.com/xamarin/xamarin-android/projects/1

The idea: use `jnimarshalmethod-gen.exe` to generate
*JNI Marshal Methods* at packaging time, so that we can avoid using
as much reflection for method registration, and *entirely avoid*
using `System.Reflection.Emit` for marshal method registration.

The problem: The Java type `mono.android.TypeManager` is Just Like
every other `Java.Lang.Object` C# subclass, in that it's a Java
`native` method that is registered with a C# delegate via
`JNIEnv.RegisterJniNatives()`.

*Unlike* every other `Java.Lang.Object` subclass, it wasn't actually
a `Java.Lang.Object` subclass. It Just Happened To Work because it
`JNIEnv.RegisterJniNatives()` used Reflection to find the
registration methods/etc., but it was otherwise "unique".

Being unique was fine, but in our desired new
`jnimarshalmethod-gen.exe` World Order™, being unique *isn't* fine;
it means it won't participate in packaging-time marshal method
generation, thus requiring use of `System.Reflection.Emit`, which
is undesirable.

Add a new internal `Java.Interop.TypeManager.JavaTypeManager` type to
use as the C# peer for `mono.android.TypeManager`, make
`TypeManager.JavaTypeManager` a normal `Java.Lang.Object` subclass,
then update `mono.android.TypeManager` to register
`Java.Interop.TypeManager+JavaTypeManager`. This removes the
"uniqueness" of `mono.android.TypeManager`, allowing it to be
processed normally by `jnimarshalmethod-gen.exe`.

src/Mono.Android/Java.Interop/TypeManager.cs

@@ -358,5 +358,20 @@ public static void RegisterPackages (string[] packages, Converter<string, Type>[
 				}
 			}
 		}
+
+		[Register ("mono/android/TypeManager", DoNotGenerateAcw = true)]
+		internal class JavaTypeManager : Java.Lang.Object
+		{
+			[Register ("activate", "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/Object;[Ljava/lang/Object;)V", "")]
+			static void n_Activate (IntPtr jnienv, IntPtr jclass, IntPtr typename_ptr, IntPtr signature_ptr, IntPtr jobject, IntPtr parameters_ptr)
+			{
+				TypeManager.n_Activate (jnienv, jclass, typename_ptr, signature_ptr, jobject, parameters_ptr);
+			}
+
+			internal static Delegate GetActivateHandler ()
+			{
+				return TypeManager.GetActivateHandler ();
+			}
+		}
 	}
 }

src/Mono.Android/java/mono/android/TypeManager.java

@@ -13,6 +13,6 @@ public static void Activate (String typeName, String sig, Object instance, Objec
 		String methods = 
 			"n_activate:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/Object;[Ljava/lang/Object;)V:GetActivateHandler\n" +
 			"";
-		mono.android.Runtime.register ("Java.Interop.TypeManager, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", TypeManager.class, methods);
+		mono.android.Runtime.register ("Java.Interop.TypeManager+JavaTypeManager, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", TypeManager.class, methods);
 	}
 }