It will depend on the head unit manufacturer as to whether car speed can be seen and passed along to AA. However this is only one way, that is AA may be able to read it, but it will not be able to set a car's speed
Yes the last thing that I'd want is AA to be pressing the gas pedal!
As for determining the speed, your phone has GPS and an accelerometer right? As such it can determine the speed all on it's own. The only question then is will it display it.