Pass along task ID when attempting a problem.

Test Plan:
Make a local edit to print out task_id in attempt_problem on the server.
Do a problem in an exercise when started from the new learning dashboard,
     see that a task ID was printed out
Do a problem in the same exercise in a tutorial, see that nothing was printed
    out

Reviewers: alpert, marcia

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2154