Re-schedule bg stop on shorter timescale

If a background user was scheduled to stop, but we postponed it, we were
using the default automatic stopping time to decide how long to postpone
it. However, that was no longer ideal because:
* we now also have background user stops due to impending alarms. So
  even if the default scheduling is disabled, we still want to be able
to postpone schedules. Without this cl, we would simply never stop the
background user; now, we will.
* the default value might be a very large value. If we already wanted to
  stop the user, but just couldn't due to some transient issue (with no
evidence that the user is actually useful right now), we shouldn't have
to wait so long when rescheduling.

Bug: 401340391
Bug: 330351042
Test: UserControllerTest
Flag: android.multiuser.schedule_stop_of_background_user
Change-Id: I46d53e5c139dbaab298714c06fdf485ceebb8b77