FlutterBinding: Create outline for platform event wire ups

[adamped]

The Binding library will likely need events to and from the Shell. This is seen as shown in the current Shell of Flutter.

Examples:

https://github.com/flutter/engine/tree/master/shell/platform/android/io/flutter/app

https://github.com/flutter/engine/tree/master/shell/platform/android/io/flutter/view

[francedot]

I'm on it