Fix the cleanup logic for the Android render thread

On Android the exit logic goes through `Godot#onDestroy()` who attempts to cleanup the engine using the following code:

```
runOnRenderThread {
	GodotLib.ondestroy()
	forceQuit()
}
```

The issue however is that by the time we ran this code, the render thread has already been paused (but not yet destroyed), and thus `GodotLib.ondestroy()` and `forceQuit()` which are scheduled on the render thread are not executed.

To address this, we instead explicitly request the render thread to exit and block until it does. As part of it exit logic, the render thread has been updated to properly destroy and clean the native instance of the Godot engine, resolving the issue.

platform/android/java_godot_wrapper.h

@@ -68,6 +68,7 @@ private:
 	jmethodID _get_input_fallback_mapping = nullptr;
 	jmethodID _on_godot_setup_completed = nullptr;
 	jmethodID _on_godot_main_loop_started = nullptr;
+	jmethodID _on_godot_terminating = nullptr;
 	jmethodID _create_new_godot_instance = nullptr;
 	jmethodID _get_render_view = nullptr;
 	jmethodID _begin_benchmark_measure = nullptr;
@@ -85,6 +86,7 @@ public:
 
 	void on_godot_setup_completed(JNIEnv *p_env = nullptr);
 	void on_godot_main_loop_started(JNIEnv *p_env = nullptr);
+	void on_godot_terminating(JNIEnv *p_env = nullptr);
 	void restart(JNIEnv *p_env = nullptr);
 	bool force_quit(JNIEnv *p_env = nullptr, int p_instance_id = 0);
 	void set_keep_screen_on(bool p_enabled);