listenbrainz: use LB defined fields for client and source info

The submission was using the unofficial fields listening_from (for the client being used) and source (for the playback source). In the meantime LB specs have formalized this more, see https://listenbrainz.readthedocs.io/en/latest/users/json.html#payload-json-details

Also include the duration. This might be used by ListenBrainz to improve matching the submitted track to MusicBrainz and also for deciding on how to keep the "playing now" notification active.