[Multi-user] Clear calling identity for backup activation

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