Bluetooth: Audio: Media control service

This commit adds the Media Control Service, and a dummy media player,
from the topic-le-audio branch.  This is a part of the upmerge of the
le-audio media control files.

This service has been developed and maintained over a couple of years
now, and is mature.  During the development it has passed both
IOP-testing and PTS qualification testing.

The commit is a pure copy of the files and content in the
topic-le-audio branch, with the following exceptions:

- files are in bluetooth/audio instead of bluetooth/host/audio, with
  some include paths updated as a consequence
- as a consequence, CMake files and Kconfig files updates are done in
  other locations
- copyrights have been updated

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>