Building for Android has changed from using a "pre-canned" build, to building with the same standard 'gradle-based' build process that Android Studio projects use. This makes it easier to edit the native Android code; after the upgrade you can now open the android folder in Android Studio, and it will know how to build the project, and will allow you to add and edit java code.
With this, we have removed the old legacy build support. Follow these steps to upgrade your existing projects to build with gradle: