Don't restore notification channel if its sound is unavailable

If the sound we're trying to restore is unavailable on the device, we
set the sound for that channel to system default. Instead, skip restore
of that channel altogether.

Bug: 158351264
Test: 1. Make sure the notification sound in Calendar is set to 'App
         provided sound' -> Run backup -> Wipe the device -> Restore ->
	 Make sure notification sound is still 'App provided sound'.
      2. Set notification sound  in  Calendar to some pre-bundled sound.
         -> Run backup -> Wipe the device -> Restore -> Make sure
         notification sound for Calendar is restored correctly.
Change-Id: Id0d0a762b381ca1ef7f4cb8305f03e9716282161