Android JNI asset extraction support (API 9+)

Signed-off-by: Filip Wasil <filip.wasil@tieto.com>
My intention is to provide a facade for android native app. Introducing a ANativeActivity as a parameter to my IOSystem seemed to be the most natural choice for me. I did not intend to provide a generic API for all java asset managers out there. 

https://github.com/assimp/assimp/pull/398

I gave another try.