mpris: Only expose the players that can play

mpris: Only expose the players that can play

In mpris we only emit "player-added" for players that can actually play,
but during NotificationBox and MessageList initialization we add them all,
including those not having being technically added.

And this may be a problem when locking the screen, since we do not want
players to show if they cannot play.

So, be consistent and expose publicly only the players that can play

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4183>