alarmmanager: Reduce calls to elapsedRealtime()

 * The RTC poweroff wakeup code needs to do an exhaustive search for
   matching alarms during every rebatching. This loop was calling
   elapsedRealtime() for every alarm over and over, which is slow on
   32-bit ARM. This results in ANRs. Just call it once.

Change-Id: I9df947d5a5c4f1d582238be63ce7da580aa26504