Fix alarm throttling.

Alarm throttling wasn't happening if the queue became empty before a new
alarm was scheduled. This fixes that so the min time between alarms is
always enforced.

Bug: 288975061
Test: atest FrameworksMockingServicesTests:AlarmQueueTest
Change-Id: Id01bcf4e7d53ae7d89c99cb7f97b86163c43ea88
(cherry picked from commit ef46dcbc)