DeskClock: 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.

Based on this patch, and adapted for CM:
https://android-review.googlesource.com/#/c/147815/

Change-Id: Ifa7b85ab93583afaf4a248e13d726997b68822ed