Dropdown widget bandaid for not rendering selected in props

Summary:
We're doing some very scary react things in our
renderer react with widgets state, where we're actually
resetting all of the answer area widgets' props when a
hint is taken.

Really, this means the dropdown should reset to unselected
(as that's what it's props think happened), but we don't
actually want this behaviour.

This change makes the dropdown ignore its props and instead
look at its DOM state when calling toJSON(), the same way
input-number does. I am reasonably confident that this
isn't toooo horrible right now, because it's what
input-number does.

Not-so-long term, both dropdown and input-number should use
their props state correctly, and the renderer in the answer
area shouldn't clear all its widgets' props when a hint is
taken.

Previous discussion: D5891

Test Plan:
 * Make a question with only a custom answer area, with
   a dropdown and input-number boxes, and two hints
 * choose the correct dropdown but put incorrect answers
   in the input-number boxes
 * click check answer
 * take a hint
 * put the correct answers in the input-number boxes
 * click check answer

Ensure that this does not succeed before this change,
but succeeds afterwards.

Auditors: alex