Add notification for alarms and timers fired from background user

When an audio source from background user gains audio focus for USAGE_ALARM, show notification to the foreground user letting them know what user the sound came from and providing actions to switch to that user and mute sound.

Bug: 314907186
Test: atest BackgroundUserSoundNotifierTest
Flag: android.multiuser.add_ui_for_sounds_from_background_users
Change-Id: Ia8f15ea847192d0db08eaf0cb20f453583a44a58