DeskClock: Handle alarm playback when the uri is not valid

- If the user deletes the file that the alarm URI is pointing to
recover gracefully by falling back to system default alarm when
it's time to play the alarm.
- This was already fixed if the user returns to the app before the
alarm goes off and checks the UI, but if the user never goes back
to the app after deleting the file, the playback is broken (will
default to silent).

Change-Id: I3896ae37b4c8252ba8d55d7660696a01cdc4d4df