Customizer: Improve `ColorControl`'s `wpColorPicker` to update UI based on setting changes.

Update Twenty Fifteen's `colorScheme` control to properly interact with the API, using `wp.customize.control()` instead of traversing DOM for other controls' container elements and stop manually updating color control UIs.

props westonruter.
fixes #30031.

git-svn-id: https://develop.svn.wordpress.org/trunk@30126 602fd350-edb4-49c9-b593-d223f7449a82