Encore is an open-source music app for Android allowing you to listen to music from multiple plug-in sources, make cross-platform playlists, create automated mixes, recognize music, and much more. It also supports casting to Chromecast, and contains an Android TV UI.
If you are interested in making plug-ins for Encore, you should check out the provider libary SDK at https://github.com/fastbootmobile/encore-providerlib.
Encore Music is licensed under GPL. Check LICENSE.md file for more informations about the main app and the plug-ins licenses.