No single-radios with multiple correct choices

Summary:
When switching a multiple select radio to a single select radio,
previously the props would not update the answer, so a question
could be saved expecting multiple choices to be checked, but
only giving the student the ability to check one.

This commit clears all checked choices if more than one is checked
and a multiple select is changed to a single select.

Fixes the cause of https://github.com/Khan/khan-exercises/issues/151197

Test Plan:
Substate and open http://localhost:8080/devadmin/content/items/x97c0c741
 * Preview it and make sure it is answerable with a multiple radio
 * Randomize the order and make sure the correct answer still works
 * Uncheck "Allow multiple selections" and make sure that all of the
   choices are unselected
 * Save the question and make sure you get an alert about not
   having selected an answer.
 * Choose an answer, randomize, and preview, and make sure the
   question works.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5810