Mullvad's not really in the business of developing for Alphabet. but any of us could though, sure.

Android is open source, but the codebase is massive and unapproachable. I managed to make some tweaks to Android, and compiled my own custom version (for one, I removed the stupid blur from lock screen album art), but I'm fairly confident I wouldn't be able to even find all the relevant code for DNS and VPN interactions in any reasonable amount of time.