Don't consider usedWidgets in clearing widgets

Summary:
Previously the first interaction of a widget was considered updating it,
so it would get cleared. This ignores those changes.

Test Plan:
* Add an input-number
* Should successfully insert the first character you add

Reviewers: jack

Reviewed By: jack

Subscribers: alpert, brianmerlob

Differential Revision: http://phabricator.khanacademy.org/D7447