Added png libs for x86 and arm ABIs

Added libs in an attempt to be able to debug a segfault near
mjPhysics. These new libpngs come from https://github.com/julienr
/libpng-android , and the modifications to the *.mk to allow for
multiple
architectures come from http://blog.marcingil.com/android-ndk-link-
static-library-to-your-jni-library/ , with infinite thanks to
both of them