Extend the threading library to back thread with an event queue.

This change allows arbitrary functions to be called on a given
thread which will clean up much of the dispatch code in bluedroid.
Looking forward, this code will be extended to allow additional objects
and queues to be attached to a thread for more customizable dispatch.

Change-Id: Id3a16256c264e3d35e6db5a562cb0e7762676457