VLCFullscreenMovieTVViewController: Fix artwork display for audio playback

This merge request brings a fix to:

• The metadata updates that are now dispatched asynchronously in the main queue when the media player state has been changed in the playback service.
• The artwork views that are now properly updated when:
  • The media's artist changes
  • The metadata thumbnail is updated and is not the same as the displayed artwork
  • The next media's starts playing

Closes #1748 (closed)

This merge request also brings a fix to:

• The artwork image view content mode that is now set to .scaleAspectFit in order to properly display the thumbnails no matter their initial sizes.

Before

After