Here's how I was able to activate Android Auto on my 2016 Mercedes-Benz A 220d (w176). The same instructions can also be used to activate Apple Car Play.

When I first got the vehicle, I was quite disappointed that it didn't come with Android Auto, and I had planned to update the head unit to one of those 10" Android displays. Luckily I stumbled upon some additional settings that I hadn't seen before while trying to figure out something else, and to my delight, Android Auto could actually be activated and works great!