Return Control Center to the system when not needed anymore

[defagos]

Description

This PR improves Control Center integration by returning Control Center to the system once Pillarbox does not need it anymore. This allows any other app previously integrated with it to automatically claim ownership again.

Previously Control Center integration was preserved during item transitions. This ensured that Control Center was never returned to another app when in use, especially when navigating between playlist items from the Control Center itself. This approach was too greedy, though, and we never really properly relinquished ownership once the underlying queue player has consumed all its items.

Before	After
cc_before.mp4	cc_after.mp4

Changes made

• Distinguish between item transitions (where Control Center should never be totally empty) and playback end (at which point the Control Center should be returned to the system) by observing queue updates. If there is no current AVPlayerItem this means playback is not currently active and Control Center must be returned to the system.
• Make this distinction obvious by replacing the NowPlaying struct with an enum (so that the totally empty state is now clearly identified).
• Remove replay support from the Control Center play button. When playback ends the Control Center is returned to the system, so replaying is not possible anyway.
• Avoid systematic insertion of MPNowPlayingInfoPropertyIsLiveStream in user info. This trick was used to preserve Control Center integration (which requires a non-empty dictionary). Instead, we now either use a totally empty dictionary (playback end) or a dictionary with at least an empty MPMediaItemPropertyTitle (when content is loaded in between items).

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).