I've tried that. The problem is I can't install directly the unsigned apk as I was using lucky patcher to modify the unsigned apk in order for it to be successfully installed and when I tried to overwrite it over on the google play dragon city, it just won't let me install it.Do not immediately install unsigned apk and play the game.
1. Uninstall unsigned apk
2. download dragon city from google play
3. connect with facebook
4. overwrite google play apk with unsigned apk through direct install without removing the original app