See, the whole premise behind Android Auto is to detach you from your smartphone and only enable you to access a couple of essential apps and features with as few interactions and distractions as possible. Counter-productive to that is the new "Minimize" feature, which essentially transforms the app into a floating chathead and allows you to use your device as you would normally do. No limitations, and the full expanse of distractions that can be oh-so-dangerous while driving your vehicle down the congested highway.
Still, the new feature might in mighty handy should you want a passenger to interact with your device, be it to key in directions to a new spot or enqueue some Kanye West on the audio system.
Minimize seems to be rolling out in waves, but you need to be sporting Android Auto v.3.3.582064 or newer in order to be eligible for its arrival. To access the feature, you need to open the overflow settings menu of the app and voila, it should be there.
Be careful while driving, please.