Correctly reschedule alarms on system time change.

Reported issue: AlarmManager would not properly reschedule repeat alarm
if the user manually adjusted date/time to before the next scheduled
instance of the alarm.

Steps to repro issue:
1.Set an alarm for 18:00 when system time is 19:00.
2.Modify the system time to 17:50
3.Note that alarm does not fire at 18:00

Thanks to Guotao Deng for identification and initial fix.

Change-Id: I5f0b4101039471182aa5134f192650821ed5d84f