Don't show letterbox UI for apps that show wallpaper.

Also, integrate hasWallpaper in mOccludesParent and occludesParent() in ActivityRecord.

Fix: 181976470
Test: manual + atest SizeCompatTests + adb shell dumpsys activity top-resumed
Change-Id: Ibc8423910223e8c4abd3641a36793477405d04a5