BroadcastReceiver in UserBackupManagerService can crash system_server

Make sure mBroadcastReceiver is initialized after its dependencies,
i.e. mTransportManager.

Bug: 130408863
Test: 1) atest RunBackupFrameworksServicesRoboTests
      2) atest CtsBackupTestCases
      3) atest CtsBackupHostTestCases
      4) atest GtsBackupTestCases
      5) atest GtsBackupHostTestCases

Manual test:
1) Before fix: Add Thread.sleep() before mTransportManager is
initialized in constructor and tirgger PACKAGE_CHANGED event. Verify
broadcast receiver callback is triggered and systen_process crashes.
2) After fix: Add Thread.sleep() before mTransportManager is
initialized in constructor and tirgger PACKAGE_CHANGED event. Verify
broadcast receiver callback is not triggered.

Change-Id: If1628628176a08a2d33d020ce270de92b606d6df