More precise control of batch alarm.

Use setWindow instead of setRepeating for alarms.
SetRepeating is inexact and the alarm can be triggered quite randomly,
while setWindow gives more precise control of trigger time.

Also unregister alarmReceiver during cleanup to avoid leak of
IntentReceiver.

Bug: 16689485, Bug:16826684
Change-Id: I5245e1424aad3c11ab5ce8a7393031e9d4f1dd03