macos: fix media playback

Given playback was stopped, many issues happened when the playback was resumed from the main controls bar.

The media time/progress slider wasn't visible anymore and app crashed unexpectdly when picture in picture was activated.

This was caused by the _currentMedia var that was set to nil when playback is stopped but never reassigned when playback was resumed.

This change will reassign the var with the current played back input item from the vlc player when the currentMedia getter is called.