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

src/wp-admin/js/customize-controls.js

@@ -869,6 +869,11 @@
 					control.setting.set( false );
 				}
 			});
+
+			this.setting.bind( function ( value ) {
+				picker.val( value );
+				picker.wpColorPicker( 'color', value );
+			});
 		}
 	});