Well a couple things about this. The base configuration for android auto IS, screen mirroring. That is the core functionality of the app that you run on head unit. Keep in mind though that, that app is connected to the CMU, and that CMU is connected to your microphone, your rocker switches on your steering wheel, and the mic trigger button along with whatever else the manufacturer decides to tie into the system. I don't have the dev kit. But I have to imagine its child's play and that there is probably some way of getting a signal from the mirroring app to talk to the OS on the CMU to display navigation info on the secondary display on the dash.
However lets start off with the basics with Mazda and hope they are going to at least do this. Anything more I'd consider icing on the cake.
One thing. You mentioned that AA is available to run directly on the phone. I've had people tell me that AA integrated into the car is now no big deal since you can mount your phone or tablet and away you go.
Did you notice the accuracy of your GPS when turning? That the system was spot on when moving and turning? One thing that AA does and why I've got a serious tech boner for AA is it uses the GPS receiver that is in the car instead of the wimpy one on the phone. The Car's GPS is not limited to a handset who's main goal is battery life, and pocketability. It is FAR more powerful and accurate. So if your car has AA and built in GPS it will use that receiver.
Google Maps with a high precision receiver who is always up to date? Yes please.
I think if it is possible, you can try Android Auto on a car and see how it works.
Do you own an android phone? If so, try to enable "Ok Google" detection feature. You do not have to touch your phone. Just say "Ok Goolge".
Same for Android Auto, you do not have to touch any button on your car at all to search for address, restaurant etc... You do not have to remember any complex structured commands like you do with traditional Infortainment system. Of course, you can still use car Mic button to active Google voice search.
People keep saying it is basically mirroring. It is more than that. You can easily buy a miracast box to mirror your phone screen to car CMU display. But that solution is what it is, screen mirroring. CMU display becomes an external monitor for your phone and that is it. You can not interact with your phone directly through CMU display. Actually, there is a tweak just for that. It is CastScreen.
With AA, you can use your car infortainment display like tablet.
For GPS signal, I have no idea when I connect my phone to Mazda Connect, Google Maps will use my phone built-in GPS system or car GPS system. I think Google Maps will use GPS signal from my phone.
My friend also tried Apple Car on the same car that I tried Android Auto. Funny, turn by turn navigation was not display on driver information display.
I installed Android Auto tweak on my Mazda CX5 earlier today. It is not official from Mazda and is slow but usable. Google Map worked as expected, easy to use. Other Android Auto apps work just fine. One main problem is I can not receive phone call through car bluetooth hands free like I used to do. A big draw back to me!
Different people have different needs but I will be very happy if Mazda roll this out soon and I hope they do not charge extra fee for this. (rei)