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.

Change-Id: I087cbe10280d4a60aa5ccfaefe24a223523fb3f2
aapt: silence these warnings

to show them, set SHOW_MANIFEST_WARNING

Change-Id: I8fd3617e03ca535df1ae0ebdb3dde3a67eb2737b