Game timer keeps running after the last challenge is answered

[pixelzoom]

Game timer keeps running after the last challenge is answered, and doesn't stop until you press Next button. It should stop immediately after answering the last challenge.

[pixelzoom]

Fixed. The game timer now stops as soon as the user has made their final guess on the last challenge. If their final guess was correct, they'll see a "Next" button that takes them to the "level completed" screen. If their final guess was incorrect, they'll see a "Show Answer" button before seeing the "Next" button. They are not charged for time incurred in reviewing an incorrect answer to the final challenge.

[pixelzoom]

I'm going to mention this issue in dev testing for graphing-slope-intercept (phetsims/tasks#812), since that's how it was uncovered.

@phet-steele Could you please review this in master?

[phet-steele]

Fixed.