Fixed game stats bug and made restart faster

The game stats were getting reset after each game.

The restart button used to save to a file which caused the game to lag.
Now the files are only update in onResume, onStop, and when the game is
lost.
Fixed 2 corner mode bugs

Remove low tiles in corner mode can result in no moveable tiles. Caused
when all tiles are 2 and 4. Fixes #25

Game not detecting loss in corner mode. Fixes #23