Refactor some Android build things for gltfio.

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().

android/gltfio-android/src/main/cpp/AssetLoader.cpp

@@ -32,18 +32,6 @@ using namespace utils;
 extern void registerCallbackUtils(JNIEnv*);
 extern void registerNioUtils(JNIEnv*);
 
-jint JNI_OnLoad(JavaVM* vm, void*) {
-    JNIEnv* env;
-    if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK) {
-        return -1;
-    }
-
-    registerCallbackUtils(env);
-    registerNioUtils(env);
-
-    return JNI_VERSION_1_6;
-}
-
 extern "C" JNIEXPORT jlong JNICALL
 Java_com_google_android_filament_gltfio_AssetLoader_nCreateAssetLoader(JNIEnv*, jclass,
         jlong nativeEngine, jlong nativeProvider, jlong nativeEntities) {