Add press state for checkboxes

Summary:
{F58250,size=full}

This looks super weird because the checkbox stays checked or unchecked as you hold, but I think updating that behavior will take longer than it makes sense to spend right now.

If you've got suggestions for making this more fluid/less jarring, let me know!

Test Plan: hey, look! (also make sure radio buttons still work)

Reviewers: marcos, mgp

Reviewed By: mgp

Subscribers: aria

Differential Revision: http://phabricator.khanacademy.org/D13245