Android code is based on the rosjava client and core libraries and has been recently upgraded to work seamlessly with android studio and the new google supported android gradle plugins.
Official source code for android projects can be found on github in the rosjava organisation.
Android development is supported via Damon's rosjava implementation.
Overview : been some large jumps made in the last few months, details here.
Official Packages : list of all official and semi-official packages in the rosjava-android ecosystem.
Gradle Plugins : what the ros-android gradle plugins are doing.
Android Studio : using the new studio to compile, hack and deploy your projects.
FAQ : frequently asked questions.