[Java.Interop] Fix JNI signature generation (#249)

Fixes: dotnet/android#1149

Commit cb161d2 broke the use of `[Java.Interop.ExportAttribute]` and
`[Java.Interop.ExportFieldAttribute]`.

Repro:

 1. [Create a class with an `[Export]` or `[ExportField]` method][0]:

		partial class MyParcelable : Java.Lang.Object, IParcelable {
			[ExportField ("CREATOR")]
			public static MyParcelableCreator InitializeCreator ()
			{
				/* ... */
			}
		}

 2. Launch the app, and cause the `MyParcelable` Java Callable Wrapper (JCW)
    to be initialized:
    
		// Will eventually cause JCW MyParcelable.class to be init'd
		var whatever = new MyParcelable();

Expected result: It works!

Actual result:

	System.InvalidOperationException: Specified managed method 'InitializeCreator' was not found. Signature: ()Lmd5<<whatever>>/MyParcelableCreator;
	  at Android.Runtime.JNIEnv.RegisterJniNatives (System.IntPtr typeName_ptr, System.Int32 typeName_len, System.IntPtr jniClass, System.IntPtr methods_ptr, System.Int32 methods_len) [0x001d2] in <36a2bb1be1a64b51b22cbef15eee5437>:0 
	  at (wrapper managed-to-native) Java.Interop.NativeMethods.java_interop_jnienv_alloc_object(intptr,intptr&,intptr)
	  at Java.Interop.JniEnvironment+Object.AllocObject (Java.Interop.JniObjectReference type) [0x00027] in <8e80eb3ca41f4c3eb745bd7c73458796>:0 
	  at Java.Interop.JniType.AllocObject () [0x0000c] in <8e80eb3ca41f4c3eb745bd7c73458796>:0 
	  at Java.Interop.JniPeerMembers+JniInstanceMethods.StartCreateInstance (System.String constructorSignature, System.Type declaringType, Java.Interop.JniArgumentValue* parameters) [0x0003e] in <8e80eb3ca41f4c3eb745bd7c73458796>:0 
	  at Java.Lang.Object..ctor () [0x00034] in <36a2bb1be1a64b51b22cbef15eee5437>:0 
	  ...

[0]: https://github.com/xamarin/monodroid-samples/blob/f944f42001fa134915069fa7bbb9cb493bfd446d/ExportAttribute/ExportAttributeTest/MainActivity.cs#L52-L86

The `InvalidOperationException` is thrown from
[`JNIEnv.RegisterJniNatives()` for `__export__` methods][reg], which
requires that `JavaNativeTypeManager.GetJniSignature(MethodInfo)`
return the same value as what the Java Callable Wrapper contains.

[reg]: https://github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Android.Runtime/JNIEnv.cs#L172-L178

Rephrased: we require that the Reflection-based
`JavaNativeTypeManager.GetJniSignature(MethodInfo)` generate
identical values as the Cecil-based
`JavaNativeTypeManager.GetJniSignature(MethodDefinition)`: the former
used by `JNIEnv.RegisterJniNatives()`, the latter by
`JavaCallableWrapperGenerator`. These *must* be consistent.

The problem is, they weren't:
`JavaNativeTypeManager.ToJniNameFromAttributes(Type)` -- called by
`JavaNativeTypeManager.GetJniSignature(MethodInfo)` and others -- was
erroneously updated to check for custom attributes on the specified
`Type` *and all base classes*. Checking base classes would eventually
cause `Java.Lang.Object` to be checked, causing *everything* to become
`java.lang.Object` unless explicitly overridden in the inheritance
chain (e.g. `Activity`, which has its own `[Register]` attribute).

The result is that `JavaNativeTypeManager.GetJniSignature(MethodInfo)`
for `MyParcelable.InitializeCreator()` was returning:

	()Ljava/lang/Object;

while `JavaNativeTypeManager.GetJniSignature(MethodDefinition)` was
generating:

	()Lmd5<<whatever>>/MyParcelableCreator;

Because these didn't match, `InvalidOperationException` was thrown.

Fix `JavaNativeTypeManager.ToJniNameFromAttributes(Type)` so that
base classes are *not* searched for custom attributes, restoring
prior behavior.

Author: radekdoulik

src/Java.Interop.Tools.TypeNameMappings/Java.Interop.Tools.TypeNameMappings/JavaNativeTypeManager.cs

@@ -261,8 +261,8 @@ static string GetSpecialExportJniType (string typeName, ExportParameterKind expo
 		// Keep in sync with ToJniNameFromAttributes(TypeDefinition)
 		public static string ToJniNameFromAttributes (Type type)
 		{
-			var a = type.GetCustomAttributes ().OfType<IJniNameProviderAttribute> ().FirstOrDefault (j => !string.IsNullOrEmpty (j.Name));
-			return a == null ? null : a.Name.Replace ('.', '/');
+			var aa = (IJniNameProviderAttribute []) type.GetCustomAttributes (typeof (IJniNameProviderAttribute), inherit: false);
+			return aa.Length > 0 && !string.IsNullOrEmpty (aa [0].Name) ? aa [0].Name.Replace ('.', '/') : null;
 		}
 
 		/*