Framework side of issue #7302511: GCM client needs to use new framework API...

...to fix background->shutdown delivery race

Add ACTION_STOPPING and ACTION_STARTING to allow such apps to keep track of
which users are started/stopped, and be involved in the process of stopping
a user.

Also get rid of the scale part of the wallpaper transitions, since it seems
like I have still failed at getting the user switch to wait until the new
wallpaper is displayed.

Change-Id: If7e8fdae3544a9d7987a1b9274dc8b49022f6f62