Make sure the full-data backup queue is properly set up on startup

Ensure UserBackupoManagerService.readFullBackupSchedule() returns
non-null container object, instead of a null object. The other areas of
the code assume that the object is valid.

Bug: 269674005
Test: 1. (Manual) Happy case:
         Verified device does not crash when the schedule file version
         is the same as the expected value.
      2. (Manual) Corrupted case:
         Verified device does not crash when the schedule file version
         is forced to NOT match the expected value. An exception was
         thrown, and caught by the corresponding catch block. The
         corrupted file is deleted, and a new one is created.
      3. atest PerformFullTransportBackupTaskTest

Change-Id: I6c28e40f164023ea23e0aebc1aa1c2d564d9ed10