Bluetooth media: Add cleanup() and avrcp object

* Add a cleanup() corresponding to initialize().  Note: we may
initialize automatically when we start the stack in the future.
* Add and initialize the default Avrcp object on topshim.  A2dp depends
on Avrcp running in some places.

Test: audio streaming
Tag: #floss
Bug: 190730870
Change-Id: I0d9ea23d2d7a36c3eb39465357bb2d70162d7799