Nonfree (SURF, SIFT) in OpenCV 3.1 for Android

edit

How can I compile the opencv_contrib files to use via JNI in android? (on Windows)

I have OpenCV 3.1 for Anroid working via native JNI and Java as well, the .so files generate fine.

I'd like to know how I can generate, I'm guessing, a opencv_contrib.so library.

Thanks!