service/mpris: fix renamed desktop entry for Media Player
mediaplayer-app is renamed as lomiri-mediaplayer-app, so follow that. This is important for e.g. ayatana-indicator-sound which requires valid desktop entry before it will recognize an MPRIS instance.
Ideally we shouldn't hardcode desktop entry here but rather look up the desktop entry for the client. But I guess that won't be simple (especially for non-Click apps).