There is recently involvement of one community member to get APwidgets going again: Using Sound in Android
However, the owner of the repo might not be around supporting this project anymore. If you want to add buttons, have you consider using Android widgets instead? Check these:
On-screen buttons without APWidgets? - Processing 2.x and 3.x Forum
Creating Native Android Menus in Processing Android Mode - Processing 2.x and 3.x Forum
Give it a try. it is not that difficult. at least, it is easier than modifying the APwidget source code.
Kf