gltfio: expose getMaterialInstances to Java.

Tested locally by hacking gltf-viewer on Android, added the following
lines to MainActivity:

```kotlin
for (mat in modelViewer.asset!!.materialInstances) {
    Log.d("gltf-viewer", mat.name)
}
```

android/filament-android/src/main/cpp/MaterialInstance.cpp

@@ -356,3 +356,11 @@ Java_com_google_android_filament_MaterialInstance_nGetName(JNIEnv* env, jclass,
     MaterialInstance* instance = (MaterialInstance*) nativeMaterialInstance;
     return env->NewStringUTF(instance->getName());
 }
+
+extern "C"
+JNIEXPORT jlong JNICALL
+Java_com_google_android_filament_MaterialInstance_nGetMaterial(JNIEnv* env, jclass,
+        jlong nativeMaterialInstance) {
+    MaterialInstance* instance = (MaterialInstance*) nativeMaterialInstance;
+    return (jlong) instance->getMaterial();
+}