DeskClock: Remove notification sound from firing and snoozing channels

* Firing alarms and timers needs high priority but we don't need a
  notification sound, as the alarm / timer already bring their own sound
* This has been lost in the rework of notification channels
  -> Needs another channel recreation

* Snoozing was lowered in priority in commit
  22b6cf2f
  but I didn't give it a new id, so existing installations are left with
  notification sound when snoozing - who would want that?

Change-Id: I5c7fc96b657e966950414b40fb61c829040ece9f