Fix answer area mistakenly resetting all widgets

Summary:
This was a tricky one to track down.

In most situations, changes never propagated up to the answer area
editor, so all was good.

In a situation where they did (like taking a hint, or anything "global") the
answer area renderer would send new props to the renderer, which would then
check if any props had changed.  Since the answer area was creating a
completely new `onChange` function every time `render` was called, any time
this happened the renderer would detect that its props had changed and would
reset all of its widgets.

Test Plan:
* do a subperseus
* create, save, and preview this item:
  * https://gist.github.com/justinj/084efb202d082fd214c3
* While previewing, check something
* Take a hint
* Your answer should remain checked!

Reviewers: alpert, jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7248