Queue: Wait for unregistration if on different thread

Before Queue object is deleted, we must make sure that the enqueue or
dequeue callback isn't executing. If on same thread, callbacks are
synchronized. If on different thread, we must wait for unregistration to
synchronize callbacks.

Test: bluetooth_test_gd
Bug: 150174451
Change-Id: Id3c980aa0bf7bd9fa10c33c5cca3751df38f7d97