Bluetooth: Audio: Rename bt_audio_unicast_server to bt_bap_...

Rename the bt_audio_audio_server API to bt_bap_unicast_server
and move API to bap.h.

This also adds the _bap_ infix for bt_unicast_server.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>