In preparation for multi window we need to refactor the FlutterView/FlutterWindow classes in dart:ui:

• Remove FlutterWindow class
• Move its logic into FlutterView and make that class non-abstract
• expose a viewId (instead of a window id like FlutterWindow does today) on the FlutterView that uniquely identifies the view

The goal is to center the engine around views, it will have no idea about windows.