Add preventDefault onTouchStarts to interactives

Summary:
On Mobile, touch events that aren't captured will be sent to the
scratchpad and cause drawings. We don't want this to happen for
interactive things, so we instead capture those touch events here.

Test Plan:
 * Check that you can still click on an input on mobile
 * Check that dragging on an input with the scratchpad open doesn't
   draw over/under it.

Reviewers: charlie, kasra

Reviewed By: kasra

Differential Revision: http://phabricator.khanacademy.org/D9891