Well, I have a bad news for you: you don't get to touch the Android Auto's UI at all.
There are only two kind of Apps you can develop for Auto,
*Audio apps
that allow users to browse and play music and spoken audio content in the car.
*Messaging apps
that receive incoming notifications, read messages aloud via text-to-speech, and send replies via voice input in the car.
For Audio app, they require you to provide a service which they can call and get information, contents from your app. (Like Delegate
in iOS?)
For Messaging app, your notifications would be shown as long as contain the CarExtender.
So.. no custom UI, no custom App.
p.s. UI are drawn on phone and send to car dashboard.