The project uses Gradle (the new Android build system) which is fully compatible with the Android Studio v.3.2. To get started download Android Studio from here and setup the Android SDK, then click Import Project.
If you need to build from the project from the command line, first you need to install Gradle:
$ brew install gradle
Then in the root directory of the project run gradle build, the APK will be available in the follwing directory helpful-android/app/build/apk