[Multi-user] Consolidate system and non-system user unlock callbacks

In Trampoline, initializing the BMS service and starting the service for
the system user are now handled separately. System and non-system users
now both use the same unlock callback to bring up their UserBMS instance.

Bug: 120212806
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest TrampolineTest
3) Unlock system user -> starts service for user
4) Multi-user enabled + unlock non-system user -> starts service for
user

Change-Id: Id49bb4a3834eb299be69d924b94a36794a4eb2e4