OS Queue and Reactor: Minor fixes

* When deleting EnqueueBuffer, unregister if needed
* Add a mutex to Reactor::WaitForUnregisteredReactable to prevent from
potential race
* EnqueueBuffer: Use an atomic_bool to synchronize Register/Unregister

Bug: 150174451
Test: cert/run --device
Change-Id: I7abcc2f763f8f0575dc0c67141571904c9a20136