Enforce BACKUP permission on Service end.

BackupManager runs in the client process, whereas BackupManagerService
runs in the system server process. Therefore, apps permissions need should
be enforced on the service side.

Bug: 158482162
Test: Manually checked that a sample app  encounters SecurityException
      after but not before this CL, when running code similar to the
      sample in http://b/158482162#comment1 to figure out whether
      isBackupServiceActive() for its own uid.
Change-Id: I59693819542a80a065a9c88373393b0ba0dbef65