Add bindings for releaseSourceAsset() and fix a double-free issue.
According to the Android Studio profiler, this makes Java memory usage
go from 18.6 MB to 6.5 MB.
We currently use static linking and will continue to do so. This means
that libfilament-jni is a subset of of libgltfio-jni, and libgltfio-jni
will become a subset of the upcoming libutils-jni.
This is a simple layer cake, but requires adding some CMake logic to the
gltfio-android CMakeLists, similar to what we already did for the
filament-android CMakeLists. This prevents an obscure CMake error about
duplicated build artifacts.
Also, previously we required clients to call AssetLoader#init(), this
is now renamed to Gltfio#init(), which is more consistent with
Filament#init().
For now this uses gltfio in the ubershader configuration in order
to avoid the filamat dependency. Note that we have not yet done a size
analysis of the `gltfio_core` library.
New Kotlin-based sample app is forthcoming.
