Implement preemptive dismiss from the alarms tab.

Bug: 20950181
Bug: 21915450

The CursorLoader now reads from a table that is a join of the Alarms
and AlarmInstances databases.
Preemptive dismiss option only appears if alarm is within 2 hours of
firing or snoozed.
Minor layout adjustments in order to accommodate the two additional
Views (one each for expand/collapse states).

Change-Id: Ieb5f70414a87e912fc631459b03788594d3425a0