Add black frame behind app window

Add black background behind letterboxed apps on displays with tall
aspect ratio. This is needed to make sure that other windows won't
bleed through status/navigation bar area, because letterboxed
windows are not considered fullscreen.

Bug: 63156733
Bug: 62893418
Test: go/wm-smoke
Test: Trigger animations to/from recents with different rotations.
Change-Id: If0eac9a3801518185d560a87d9f88ee0aafa3a9c