Fix a minor issue in hintUsed

Summary:
 #FIXIT

This fixes a minor issue where "hintsUsed" got doubled to what
it should have been while restoring, though that didn't
seem to result in any visible bugs (yet).

Test Plan:
 - go to any exercise
 - take all the hints
 - refresh the page
 - see that all hints are automatically expanded

Reviewers: joel, aria

Reviewed By: joel, aria

Subscribers: jessie, mgp

Differential Revision: http://phabricator.khanacademy.org/D13008