Reschedule stop background users if Guest current

If the current user is a Guest, don't stop background users due to their
having been scheduled for stopping-due-to-being-in-background. Instead,
reschdule their stopping for later.

The idea is: Don't kill a background user just for the sake of a Guest.

Fixes: 335739360
Test: atest UserControllerTest#testScheduleStopOfBackgroundUser_rescheduleWhenGuest
Test: atest UserControllerTest
Flag: ACONFIG android.multiuser.scheduleStopOfBackgroundUser
Change-Id: I4f08e654e608898e55c6c84d3fe07917f1b137d0