VLCRemoteControlService: Fix skip command and next index

This merge request brings a fix to:

• The skip command is now shown in the control center when there is no media list.

Closes #1532 (closed)

This also fixes the next index computation by removing the call to the isNextMediaAvailable method.

Instead of only checking if the next media is available, this method computed the next media index and updated the current index without changing the currently playing media.

Closes #1506 (closed)