Pass through the task's generation time to the /attempt API call.

Summary:
This is the khan-exercises portion of 'Track MasteryTask generation time on the
task' (D14299)

Test Plan: See D14299

Reviewers: benkomalo

Reviewed By: benkomalo

Subscribers: jace, #data-science

Differential Revision: http://phabricator.khanacademy.org/D14300