Prevents NPE after stack got removed from display

Application crashed and the top activity was immediately
finished after paused timeout. If there was no activities
left in stack, it would be removed from display and updated
the display id to invalid. Therefore, the system crashed
in accessing a null ActivityDisplay object.

Bug: 120667451
Test: existing test pass

Change-Id: Ica978faf4233d8e6a709fa92f87d850f6ab8e061