LE Audio: Improved media_proxy internal data structure

[Thalley]

We want an improved internal data structure for the media proxy

the mcc callbacks should be moved to the mprx struct

• 2022-03-02: DONE

the remote player to be guarded bt CONFIG_BT_MCC

• 2022-03-02: DONE - guarded by CONFIG_MCTL_REMOTE_PLAYER_CONTROL_BT

the player pointers to be replaced/changed so that they can be used for player existence checks, and the "registered" variables can be removed.

• 2022-03-03: WON'T DO - will not gain anything in the current implementation. May become useful for multiple player instances, if the player supplies the player struct.