The popular, open-source media player firmware Rockbox has recently been made available for Android. Rather than run as an operating system, Rockbox operates as a standalone application that you can install as usual with an APK. Development is still on-going, but Rockbox dev kugel has a few pre-compiled APKs hosted for you to try out if you’re interested.
These builds are unofficial and are purely for demonstrative purposes, but they seem to work pretty nicely. Currently the navigation/control interaction method is touchscreen only, and screen rotation is not supported - Rockbox will only run in Portrait orientation.