Expanded click area for radio content

Summary:
Radio content boxes were dropping clicks because the event was bound on the inner content
rather than the box itself. Now, the touch event is bound to the box itself to avoid missing
out on clicks.

Test Plan:
- Run mobile app.
- Go to exercise with radio (e.g., "Decomposing Fractions").
- Try clicking radio boxes, especially around the perimeter.
- Try clicking the actual checkbox.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11101