VLCPlaybackService: Fix next index computation and repeat mode handling
This merge request brings a fix to:
- The next index computation, the _currentIndex variable is now always updated when the next index is computed.
This variable is also the one being returned.
This merge request also brings a fix to:
- The current index update when the user selects a media through the play queue, it avoids losing track of the current play queue position.
- The "Do not repeat" mode that triggers the end of playback when the last item of the play queue ends playing.
Closes #1694 (closed)
Closes https://code.videolan.org/videolan/ios-support/-/issues/2498