Have Screensavers Listen to Alarm Changes

So they can update the "Next alarm..." display

Instead of listening to our own Alarms.NEXT_ALARM_TIME_SET, we observe
the system setting Settings.System.NEXT_ALARM_FORMATTED. This way we
respect 3rd party apps that use this API as well as ourselves.

Change-Id: Iaa90ba8335c38bd6b0ed4571e0784f4f8cb2d154