Given that Optional Content visibility is only intended/supported to be updated via the `OptionalContentConfig.setVisibility`-method, this patch actually enforces that now.
Note that this will be used by the next patch in the series, and will help prevent inconsistent state in the `OptionalContentConfig`-class.
*Please note:* This patch also uncovered a pre-existing bug, related to iterating through the visibility groups in the constructor, for the `baseState === "OFF"` case.