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: Id6f86435add1a1d13a0f18f3979df68a6ce16663