If you ever need to get the root data directory of your Android application (app) from within your Java code, I can confirm that this approach works:
File rootDataDir = getActivity().getFilesDir();
When I log that directory like this:
Log.i(TAG, rootDataDir.toString());
it prints this output for my application:
/data/data/com.alvinalexander.mynewapp/files
where com.alvinalexander.mynewapp is the package name for my new Android app.