b/2224837 Changed the plumbing of how alerts/notifications work.

Instead of processing each notification separately event (scheduling an alarm with AlarmManager for each event, posting notification to NotificationManager for each event, etc), only one alarm should be fired for each wake-up time. AlertService will look through the CalendarAlerts table and decides how to update the notication manager. This is more efficient and simplifies the codecode. It is also the long-term fix to elimiate overlapping notification ring tones e.g. DrooDrooDrooooid b/2194249

Also consolidated alarm scheduling code to one place.

Change-Id: If58ce278d55b28abad703f8aeee466d44952fcd0