Convert OPP notifications to use NotificationChannel

In O, Bluetooth notifications are hidden by default. This must be
changed to use notification channels so that the priority of the
notifications can be updated.

Bug: 36974285
Test: Send files/batches over OPP
      Testtracker: 84338
Change-Id: Ie34123bbb559cc760c83a6bdc7094a3a72382c33