Multiplication hints show true value of numbers in intermediate steps

Summary:
 Multiplication questions now shown without the long multiplication setup. Intermediate steps show true values of numbers.

Change how multiplication questions are shown and add final hint

Test Plan: Try all the multiplication exercises and make sure the hints make sense.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D11892