Well...guess what just came out today? PLEX for Android Auto! And, in hooking up my Moto X Pure phone, and firing up Android Auto, I tried out Google Play Music, Amazon Music and Spotify, and they have updated it since January, so now the album art is much more visible, compared to the barely visible album art (almost invisible) in CarPlay.
The only thing about Android Auto I don't like so far is it seems (maybe I'm doing it wrong) the phone is locked to Android Auto and you have to use the car screen. To use any other apps on the phone, you have to quit Android Auto.