Add the answer changed callback to the drop down widget

[jingtang10]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #652

Description
Add the answer changed callback to the drop down widget which was previously missing

Alternative(s) considered
NA

Type
Bug fix

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct
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• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach
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