Bluetooth: Audio: Media control - pass structs by reference

Changes the media_proxy API to pass structs by reference instead of by
value.

Also pass structs by reference in internal functions in media player
and in media controller test.

Add pointer checks to the public API implementation where
user/application code provides structs by reference.

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