I did a little hack for Kroniax to get access to the "NativeActivity". Just copy the "Activity.hpp" from the sfml/src/system/android directory to your project and include it in a source file. Then you can use sf::priv::getActivity(nullptr)->Activity to get the NativeActivity.
It's just a hack and I don't know if its a good one, but it worked for my game
(There could be race conditions)
I hope SFML will implement a out of box solution for this.
AlexAUT