Check if task was top of stack before it's removed

To decide if home stack should be brought foreward. If the app is
finishing, it's no longer considered "running" and cleanUpActivityLocked
could move focus to the activity below. Then removeActivityFromHistoryLocked
would fail to bring front home stack as the removed task is no longer top.

bug: 29030518
Change-Id: I887e7c226258651d887cb0a4a6c6caab0a04e6e6