Graphene1 I don't think the car gets access to any of your private data, because the entire Android Auto UI gets rendered on the phone and the screen frames are streamed to the car. The same goes for touch, audio and car sensors like fuel and compass. So no concerns here. The only way for the car to collect any personal data would be through Bluetooth (AAuto automatically connects to the car's Bluetooth for phone calls).
Just a disclaimer, that I'm not 100% confident of this, but I'm basing my answer on docs from Google and personal experience.
As @other8026 said in another topic, more details about this will be published on the website soon, so I'd recommend looking there for verified correct information