MPNowPlayingInfoCenter don't pause when playbackRate is set to 0

[XZB-1248]

I encountered an issue: when I set MPNowPlayingInfoCenter's MPNowPlayingInfoPropertyPlaybackRate to 0, the lock screen still shows the play button, but the progress no longer increases.
Eventually, I found out that when calling player.pause, the audioEngine continued to push silences (the note for playerNode.pause is "Pauses audio from the current decoder and pushes silence"), which led the system to think that audio was still playing, thus it continued to display the pause button.

This problem troubled me for several days, and after multiple tests, I found that the issue could be resolved by calling player.playerNode.engine?.pause after player.pause, and player.playerNode.engine?.start before player.resume or player.play.

Although it’s not recommended to directly manipulate AVAudioEngine, it indeed solved my problem.
I hope this can help others.

[sbooth]

It might be worth adding an optional argument to pause that would allow pausing the AVAudioEngine as well. The reason behind the way things work now is that pausing AVAudioEngine also stops the hardware, which on iOS could make sense, but with external devices on macOS isn't always ideal.