[Xamarin.Android.Tools.Bytecode] Add EnclosingMethod, SourceFile - #469

Context: dotnet#466

While looking at `class-parse --dump` output for various
Kotlin-compiled `.class` files, providing support for the
`EnclosingMethod` and `SourceFile` annotation blobs looks to be
useful.

Add support for parsing the `EnclosingMethod` and `SourceFile`
annotations.

Update the generated XML so that the `SourceFile` annotation is
avaialble from the `//class/@source-file-name` or
`//interface/@source-file-name` attributes.

Update the generated XML so that the `EnclosingMethod` annotation is
available from these new attributes on `//class` or `//interface`:

  * `enclosing-method-jni-type`: The JNI signature of the type
    declaring the enclosing method.

  * `enclosing-method-name`: The name of the enclosing method.

  * `enclosing-method-signature`: The JNI signature of the enclosing
    method.

For example, if `TestType.action()` had an anonymous inner class:

	class TestType {
	  public void action (Object value) {
	    Runnable r = new Runnable () {
	      public void run() {
	        System.out.println ("foo");
	      }
	    };
	  }
	}

This could result in the creation of a new `TestType$1` class for the
anonymous inner class, with the resulting `enclosing-*` attributes:

	enclosing-method-jni-type="Lcom/xamarin/JavaType;"
	enclosing-method-name="action"
	enclosing-method-signature="(Ljava/lang/Object;)V"