[just_audio_background] Fix: Shuffle order out of sync after inserting/removing item from ConcatenatingAudioSource

[jonmarkhall]

When calling insert() or removeAt() on a ConcatenatingAudioSource, a new shuffleOrder is passed with the request but it is not updated in just_audio_background's corresponding ConcatenatingAudioSourceMessage. This results in the shuffle order being out of sync, which can cause issues such as an out of range exception in the case of removeAt().

This PR also includes a change to just_audio which I found was necessary to resolve the issue I was experiencing with removeAt(), though I'm not sure why it seemed to make a difference.