Protect against assumptions of a top focused stack.

RootWindowContainer#getTopDisplayFocusedStack() has the potential to
return null when there are no focusable stacks. This guards against
many assumptions that a top focused stack exists.

Bug: 144085050
Bug: 148125390
Bug: 150754510
Fixes: 144085050
Test: ActivityStarterTests
Test: ActivityStackTests
Test: ActivityTaskManagerServiceTests
Test: RootWindowContainerTests

Change-Id: Ibefc3a802aba5a90898837359ec59a48f39834dc