For iOS Add2App, FlutterViewController's need to expose a way to tell the embedded Flutter view what route should be loaded. They also need to be able to communicate back to the host app that they're done, so the HostApp can perform appropriate logic to hide/dismiss the view.

This issue is tracking three different problems:

• CupertinoApp did not properly handle setInitialRoute logic (whereas MaterialApp and WidgetsApp did). Fixed in Widgets app refactor #22269
• SystemNavigator.pop did not work in some valid cases on iOS; PR in flight at SystemNavigator.pop can pop w/o UINavigationController engine#6341
• Expose push/popRoute on FlutterViewController; PR is in flight at Expose push/popRoute on FlutterViewController engine#6347

/cc @matthew-carroll