Based on this thread, there is no solution that I know of beyond going back to WP, which I just might do given this severe limitation.
http://forums.androidcentral.com/samsung-galaxy-s7/674334-ok-google-not-coming-through-car-bluetooth.html
Did you find any solutions to this? I was under the impression that Cortana for Android would have this functionality but I've never tried it (I have an Android phone now but still haven't swapped my SIM as I just don't want to give up some of the features like this). It is a feature I use with...