Stop activity onVisibleBehindCanceled()

Once an activity is no longer visible behind and has released
its background resources, it is added to the list of activities
that can be stopped, but not actually stopped until the next
major event (like another activity starting). We now schedule
the idle processing once the background resources have been
released so the activity can be stopped as soon as possible.

Bug: 18191707
Change-Id: I472eee949c1a78b4d944454463f03c90e7d2618b