Fix issue #5321282: Force Stop Button in Battery Screen Not disabled correctly

If the app had activities still finishing, when we checked whether it was
now stopped we would get told no.  Also some other improvements:

- Schedule an idle as part of the force stop, to get any finishing
  activities out of the stack soon rather than waiting for some activity
  to idle.
- Don't filter out stopped system apps.  This is dangerous because
  system apps may have no way for the user to explicitly launch them,
  so they could get put into a stopped state for which there is no way
  to get them out.  Also if the user really wants a system app to not
  run, the new disabling mechanism is more appropriate.

Change-Id: I34003f21dac29e2ca0f66a23b88c710de41bab99