settings: Move headset option, hide MoreDeviceSettings if empty

 * This option fits much better in the sound menu under "automation
   and effects", and on many devices it's the only option in the
   "More..." menu under Tap & Pay.
 * Move it, and remove the MoreDeviceSettings top-level category if
   there's nothing else in it.

Change-Id: Idd82f582a1ba54f8b45fbd4fbe631057e9515f86