VLCPlaybackService: fix shuffle behaviour

Port of a PR from Github:

This is the behaviour I have added:
Given a user has shuffle mode on
And they have just finished or skipped Song C
When the user presses the "previous track" button
Then Song C plays

Current behaviour on master:

Given a user has shuffle mode on
And they have just finished or skipped Song C
When the user presses the "previous track" button
Then a random song plays

ref: pull request 449