As part of bmgr activate, if the user is unlocked then we attempt to
start the backup service for them. However, as part of initialization,
AMS enforces the system uid so clear calling identity before starting
the service (as we have the shell uid).
Bug: 123572171
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest TrampolineTest
3) Manual: Unlock secondary user -> bmgr activate -> no exception thrown
and service started
Change-Id: I441491e19d9f3c9b49b5778d51ae5331230b129b
312e780346