I don't think you understand how Android Auto works. It's completely controlled by your phone, the car is just a renderer. Most have wired connections, more recent ones are wireless. You can't install third party apps on your head unit (unless it's an aftermarket android head unit or car PC). Spotify comes pre-installed from the factory on some head units (this has nothing to do with Android Auto) because they have $multi-billion agreements with the car makers and head unit manufacturers.