[Multi-User] Unregister stopped users in BMS

Adds a handler for system service callbacks when a user is stopped. This
currently only unregisters the user in bookkeeping but will be used in
the future for cleanup work such as cancelling scheduled jobs, etc.

Bug: 120212806
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest TrampolineTest
3) Enable multi user + register user -> service started for user; stop
user -> user is unregistered

Change-Id: Ic712da646f961b9bb02c7d77d964a13345eca7ec