add Material.compile and Engine.flush java bindings (#7348)

* Add Material.compile() Java binding.

* Add Engine.flush() java binding

* Add Scene.forEach java binding

update the Android gltf-viewer sample to precompile all variants of all
materials in the scene, similarly to the desktop sample.

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

@@ -384,6 +384,13 @@ Java_com_google_android_filament_Engine_nFlushAndWait(JNIEnv*, jclass,
     engine->flushAndWait();
 }
 
+extern "C" JNIEXPORT void JNICALL
+Java_com_google_android_filament_Engine_nFlush(JNIEnv*, jclass,
+        jlong nativeEngine) {
+    Engine* engine = (Engine*) nativeEngine;
+    engine->flush();
+}
+
 // Managers...
 
 extern "C" JNIEXPORT jlong JNICALL