iOS: playback can't be controlled from Bluetooth accessories

[jscholes]

With a Bluetooth accessory such as a headphone or speaker connected, Paperback doesn't respond to standard multimedia control commands such as play/pause and rewind/fast-forward.

Reproduce (I tested on AirPods 4):

1. Connect the Bluetooth accessory.
2. Start TTS playback in Paperback, with nothing else playing on the device.
3. Actuate the command for play/pause on the accessory (e.g. I do a single AirPod stem squeeze):
   • Expected: the book will pause.
   • Actual: either nothing happens, or another app's audio playback is resumed.
4. Actuate the play/pause command again:
   • Expected: the book will resume.
   • Actual: either nothing happens, or the audio from the app that was resumed after step 3 ends up paused again.
5. Actuate the accessory's command for seek forwards (e.g. I do a double AirPod stem squeeze):
   • Expected: the book position will jump forwards by the navigational unit selected in Paperback's UI. Playback will resume from that new position.
   • Actual: either nothing happens, or the command is handled by the other app that was previously playing audio.
6. Actuate the accessory's command for seek backwards (e.g. I do a triple AirPod stem squeeze):
   • Expected: the book position will jump backwards by the navigational unit selected in Paperback's UI. Playback will resume from that new position.
   • Actual: either nothing happens, or the command is handled by the other app that was previously playing audio.

Note: I suspect this is an amalgamation of issues #498 and #500. Apologies if this turns out to be a duplicate because fixing those fixes this, but I thought it was worth filing separately anyway as it's the number one blocker to me actually using the app today.

[trypsynth]

Should be fixed, I think, but I don't have any bluetooth accessories to try this with. Attempted to upload a new build and Apple started randomly giving me this error, so that's cool. "90426: Invalid Swift Support. The SwiftSupport folder is missing. Rebuild your app using the current public (GM) version of Xcode and resubmit it." No idea why this broke after weeks of working, me not touching my Mac and a WWDC, but it did.