This app was mentioned in 10 comments, with an average of 2.70 upvotes
If you're interested, projectM (an open-source implementation of Milkdrop) supports both Google Play Music, and Poweramp (and also can be run as a Live Wallpaper). It might become redundant at this point, but it's out there. It can load custom visualizations as well.
Project M is pretty sweet. It's technically not a player, but I'm pretty sure it uses OEM player by default. Free version exists, but it's ad heavy. Searching this turns up several other decent options as well.
https://play.google.com/store/apps/details?id=com.psperl.projectM
I'll just leave these here - - - - - ->
https://play.google.com/store/apps/details?id=com.psperl.projectM
https://itunes.apple.com/us/app/projectm-music-visualizer/id530922227?mt=8&ign-mpt=uo%3D4
You can thank me later. :D
projectM Music Visualizer for Android (https://play.google.com/store/apps/details?id=com.psperl.projectM&hl=en) has awesome milkdrop visuals that respond to music apps or microphone input. I would imagine that, if you did some kind of line in or audio interface connection into a phone, you could get the visuals to respond to your setup. I actually use this program in a pretty ghetto setup, lol: I just stick the phone microphone up to my speakers and send the visuals to my TV via Chromecast.
Milkdrop is already available on Android, it's called projectM, here you go: https://play.google.com/store/apps/details?id=com.psperl.projectM&hl=en
I'm not sure it'll work with Android Auto, but I can't see any reason why not.
edit: already been mentioned, my bad :(
They turned it into an app - Project M
https://play.google.com/store/apps/details?id=com.psperl.projectM
If you have Android, look up Project M. It's milkdrp on Android. https://play.google.com/store/apps/details?id=com.psperl.projectM
projectM Android Live Wallpaper - reacts to local sound.
just found this too https://play.google.com/store/apps/details?id=com.psperl.projectM