In `wp.media.view.Settings::update()`, when handling checkboxes, check for a value of `'false'` in addition to casting `value` to boolean. `!! 'false'` evaluates to `true`.

Props adamsilverstein.
Fixes #23954.



git-svn-id: https://develop.svn.wordpress.org/trunk@28371 602fd350-edb4-49c9-b593-d223f7449a82