Don't auto-stop bg user if it's playing audio

Under certain conditions, we may stop a background user due to
inactivity. However, that user may be "active" even if it isn't visible;
in particular, it could be playing audio (due to an alarm). In such a
case, the user isn't inactive, so we shouldn't automatically stop it.

Bug: 378575468
Bug: 330351042
Flag: android.multiuser.schedule_stop_of_background_user
Test: atest UserControllerTest
Test: had bg user sound an alarm and confirmed it wasn't stopped
Change-Id: Ifbb359afc2508c288c3a8ead67e553f45d56f71b