rtc: alarm: Update hrtimer if alarm at the head of the queue is reprogrammed

If an alarm was restarted with a value that moved it away from the head
of a queue, the hrtimer would not be updated. This would cause unnecessary
wakeups.

Change-Id: I7c6dc7e6085a1f03722b2fcb14a83987ef674917
Signed-off-by: Arve Hjønnevåg <arve@android.com>