Android: use FFI-based dlopen() and JNI interfacing

The most relevant changes are in the android-luajit-launcher project, where I refactored the dynamic linking and build a JNI interface, making the C code a lot lighter and using Lua/FFI instead. It's all within a branch ("ffi-loader") there: https://github.com/hwhw/android-luajit-launcher/tree/ffi-loader

A first test looks promising.