Issue-7: Countdown timer doesn't stop counting down [1/1]

[Problem]
1) CountDownTimer.cancel() does not cause the count down to stop.
2) Another CountDownTimer for a student is erronously created when switching tabs.

[Solution]
1) Use the fixed version of CountDownTimer from AOSP master
2) Attach the CountDownTimer to the Student. UI callbacks are set when the view is refreshed. This keeps separation of
       data and view.

The UI was also updated to split the screen horizontally in the middle in the current students view.