Media: cleanup recent changes to `media.view.FocusManager`.

* Adjusts the focus method to target the menu for the current modal
* Refactor the FocusManager so that the method names and the functionality matches more closely with how it currently works. The changes in 4.0 to the FocusManager change the purpose of the focus manager entirely — instead of being a way to save and restore focus state, the new FocusManager is about constraining keyboard navigation via the tab key to inside the modal. 

Props gcorne.
See #23560.

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