BroadcastOption for "this is an alarm broadcast"

Instead of a callback/observer mechanism between the alarm subsystem and
the broadcast delivery subsystem, there will be a direct
BroadcastOptions tag that the Alarm Manager uses to annotate broadcasts
that originate from an alarm firing.  Only the OS itself can use this
BroadcastOptions annotation.

Fixed unchecked null reference situation.

Bug: 161537193
Test: atest CtsAlarmManagerTestCases
Test: atest BroadcastOptionsTest
Test: atest FrameworksMockingServicesTests:AlarmManagerServiceTest
Change-Id: I70a7be35f198bcf9a3cfa232c309a381b2391e34