Move display to bottom only when no next focusable stack

If a display contains several focusable stacks, the display should
not lose focus when one of stacks moves to bottom.

Bug: 118488048
Test: atest ActivityStackTests#testMoveStackToBackIncludingParent
Change-Id: Idafde99b7b5cc88686ad957bd878d975867db3e7
2 files changed