Add MediaBrowser implementaion for A2DP.

MediaBrowser is a way to control playback of any generic media
application and to conform to this framework API the a2dpsink client
uses the same format.

This middleware provides a way to use A2DP via the MediaBrowserService
and a user for A2DP sink should not need to use the raw Bluetooth APIs.

Change-Id: I73b4a51acc469f503286bfd6d33a1766e30fc50d