Media keys have been taken over by iTunes

My Macbook Pro's media keys have just been taken over by iTunes. For years, I've used Amazon Music with success. Pressing the play/pause/prev/next buttons caused Amazon Music to, well, play, pause, or change tracks. It works exactly as you'd hope.

All this came crashing down this morning, and very suddenly. I hadn't updated any applications. The most recent OS update to 10.14.6 was installed last weekend -- well before this mystery problem began.

Now, when I hit play/pause, iTunes starts. I've never used iTunes on this computer. It isn't even authorized on my Apple account! After logging in, it failed to play songs, rapidly skipping from one to the next and spamming the notification area with "now playing..." boxes even though the songs aren't playable.

I can't figure out how to change things back. I've seen recommendations that I install third-party software to redirect the media key outputs. I do have Amazon Music listed and checked in System Preferences > Security & Privacy > Privacy > Allow the apps below to control your computer.

How can I change which program responds to the media keys?