Add an apiOption `interactionCallback` that's called when the user interacts
Add an apiOption `interactionCallback` that's called when the user interacts

Summary:
This is to allow some mobile UI which doesn't show up until the user has
interacted with the problem.

Test Plan: Not really testable ATM from here (see webapp diff for more)

Reviewers: kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D26453

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