Introduce JNI bindings for gltfio.

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.

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

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <jni.h>
+
+#include <gltfio/MaterialProvider.h>
+
+using namespace filament;
+using namespace gltfio;
+
+extern "C" JNIEXPORT jlong JNICALL
+Java_com_google_android_filament_gltfio_MaterialProvider_nCreateMaterialProvider(JNIEnv*, jclass,
+        jlong nativeEngine) {
+    Engine* engine = (Engine*) nativeEngine;
+    return (jlong) createUbershaderLoader(engine);
+}
+
+extern "C" JNIEXPORT void JNICALL
+Java_com_google_android_filament_gltfio_MaterialProvider_nDestroyMaterialProvider(JNIEnv*, jclass,
+        jlong nativeProvider) {
+    auto provider = (MaterialProvider*) nativeProvider;
+    delete provider;
+}