Release wakelock only when no alarms are in-flight

Fix the bug where the alarm wakelock could be released before all alarms
completed.

Test: atest FrameworksMockingServicesTests:com.android.server.alarm

Flag: com.android.server.alarm.acquire_wakelock_before_send

Fixes: 400058421
Change-Id: I86210548b9a8fc6050577ca1f7e973b127c47862