[Xamarin.Android.Tools.Bytecode] Add EnclosingMethod, SourceFile

[jonpryor]

Context: #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"

[jonpryor]

It helps if I commit & push my entire local patch...