Java callbacks are now directly dispatched to their handler, instead
of first going through filament's opportunistic dispatch, reducing
callback latency.
Replace with forward declarations if needed and includes in .cpp that
now need them.
The idea here is to have our headers have the least amount of impact as
possible on our clients (e.g. compilation time).
No need for a proper library, this is just a common location to simplify
JNI bindings in other projects like filamat and gltfio.
There is no need to move the one Java source file (`NioUtils.java`)
since downstream libraries will have a dependency on Filament, and
FindClass should work fine.
