Logic for detecting expanded Text and Media widgets is inconsistent/unreliable

[westonruter]

The Text widget and Media widgets in 4.8 defer to initialize their JS fields until the widget fully expands since the JS logic needs the element's dimensions to be finalized to initialize properly. The current logic for detecting the completion of the animation is faulty in that it is looking at the wrong element for the :animated pseudo-class and it also should be using the open class instead, the latter which is not properly getting set for widgets in the Customizer.

[westonruter]

In 41249:

Widgets: Add missing open class for expanded widgets in Customizer; utilize open class for expansion detection in Text and Media widgets.

Fixes #41625.