Keyguard: fix NPE setting lockscreen wallpaper

Continue previous patchset (check history)

------- Cherry-picked commit -------

After boot the view might not be laid out and so getWidth() and
getHeight() return 0 and will cause a NullPointerException in
createBitmap().

Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
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