Clear out callback when canceling an alarm

VSyncDispatchTimerQueue tries to cancel its timer callback when
destroying itself, but Timekeeper might still run the callback if
disarming its timer happened immediately after polling but before
invoking the callback. To avoid this race condition, delete the callback
while holding Timekeeper's mutex.

Bug: 304675978
Test: builds
Change-Id: Ifa7a4fcb65481b6a5ddfcfeb6bc7a53cb37ec168