fix issues

[andreastr99]

I've solved 3 issues:

• The Player class does not satisfy the good practices of code writing. The name and value fields must be private and must be used by getters/setters to work with them outside of the class.
• When the game starts if you press the back button you quit the app and the game must start all over. You could use the Toast class to appear a message on the screen to warn the user that he’s going to quit the app.
• Another issue that I spotted is that when you using the app you can’t start over the game unless you quit the app or finish the game. For that thing, you could add a replay button to let the user start again the game.