Fix mobile device check for numeric answers.

Summary:
Current the user agent on the client side.

Causes the numeric keyboard to show up on questions with numeric answers.

Test Plan: * Changed User Agent to Nexus 7 using User Agent Switcher on Chrome Desktop. Saw input type=number in the inspector.

Reviewers: tom

Reviewed By: tom

Differential Revision: http://phabricator.khanacademy.org/D1013