Add option for schedule_next_instance to force rescheduling

If we're rescheduling a periodic timer in the context of an alarm
expiration we want to force rescheduling to happen.

The alarm at the front of the list after scheduling the next
instance may be a different alarm. The old code would assume it was
already scheduled correctly, meaning nothing would ever get scheduled
after that.

Change-Id: I8accf5d004e69dfd6477ab7ec529eea42b3a88f8