Fix alarm query

Bug: 25895834

ag/811037 left a bug in place that this change addresses.

Specifically, if a one-time alarm is missed, an entry is left in
the alarm_instances table with a state of MISSED. The query's
join clause would not return a row for the alarm in this case.

A subselect is now used to return the alarm_instances row with
the smallest state value. This means:

 - SILENT instances are preferred over MISSED instances
 (the only case where 2 instances exist for the same alarm).
 - MISSED instances are returned if they are the only instance for
 an alarm (the above case)

Change-Id: I31de6d8b2d2bf7719f6b027a2401b1da1a09f247