improvements to EntityManagers and Filament APIs (#7302)

* prevent public classes from being created on the stack

- we used to to this by deleting operator delete, but this prevented
  the internal "F" classes from being virtual; which can be useful
  when using EntityManger::Listener.
  now we just make the destructor protected in each class.

- EntityManger::Listener now has a virtual destructor so that
  objects could be correctly destroyed from Listener*

* improve EntityManger and Component managers

- all component managers now have the same "base" API
    - getComponentCount()
	- empty()
    - getEntity()
    - getEntities()

- Scene now has getEntityCount()

- EntityManager now has getEntityCount()

- all component manager implement gc() the same way, by calling destroy()

- SingleInstanceComponentManager::gc() that calls removeComponent() has
  been removed because it's dangerous. removeComponent() is often
  not enough, some additional cleanup might be needed.

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

@@ -71,6 +71,13 @@ Java_com_google_android_filament_Scene_nRemoveEntities(JNIEnv *env, jclass type,
     env->ReleaseIntArrayElements(entities, (jint*) nativeEntities, JNI_ABORT);
 }
 
+extern "C" JNIEXPORT jint JNICALL
+Java_com_google_android_filament_Scene_nGetEntityCount(JNIEnv *env, jclass type,
+        jlong nativeScene) {
+    Scene* scene = (Scene*) nativeScene;
+    return (jint) scene->getEntityCount();
+}
+
 extern "C" JNIEXPORT jint JNICALL
 Java_com_google_android_filament_Scene_nGetRenderableCount(JNIEnv *env, jclass type,
         jlong nativeScene) {