Bluetooth: Audio: Additional MEDIA_PROXY defines

Add two MEDIA_PROXY group type defines corresponding to the MCS ones.
Update the mpl.c to use the newly added names.

The purpose of the exercise is to get a complete set of MEDIA_PROXY
defines, so that the media player can be made fully independent
(define-wise) of the MCS.

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