There's no such thing as "quit app" on Android, you can only minimize or kill it from the recent apps list.
When the game is minimized, it doesn't calculate anything and quietly resides in RAM till the time Android OS decides to kill it (memory required for other app / it thought that you'll not use this app).

And there's no such thing as "close app" buttons in Chrome, Hangouts, Google Maps, Gmail or any other normal software - because Android works in other way.