Fix frontOfTask assignment.

If all activities of a given stack were finishing, no activity was
marked as front-of-task. This confused ActivityManager, so make sure
there's always exactly one activity marked as front-of-task.

https://android-review.googlesource.com/#/c/98918

Change-Id: I087cbe10280d4a60aa5ccfaefe24a223523fb3f2
Merge "Fix frontOfTask assignment." into android-4.4