Implement alarm precedence for broadcast delivery

Rather than queueing alarm-triggered broadcasts with the rest of the
outbound ordered broadcasts, we now give them delivery precedence over
the general queue, to minimize parasitic execution under the alarm
wakelock.  A new DeviceConfig flag can optionally be used to override
the platform default and select between immediate alarm delivery and the
previous behavior.

Bug: 161537193
Test: atest CtsAlarmManagerTestCases
Change-Id: Ic18654427a07831dfbe7ec5edace36999cf9973f