Clean up starting window to prepare for saved surfaces

- Move all starting window logic to AppWindowContainerController
- Use startingView to hold any kind of contents for startingWindow
- Remove some conflicting code which looks very old and doesn't
apply anymore.

Test: Make sure starting window still works.

Bug: 31339431
Change-Id: I018dd013ab7e64a44932b6d54ae9bb4a47f315d3