Draw background color above display area...

...so the background color layer won't occludes any other windows which
does involved the transition.

Also fix the animation transformation accumulates to next change in
DefaultTransitionHandler#applyTransformation

Bug: 283050044
Test: follow issue description, when launch a new task on non-top task,
the background color of animation won't occludes the lower task.
Test: atest ShellTransitionTests StageCoordinatorTests

Change-Id: I02cd3a232350a1dc74e755f9445179ccb216507f