Add-button: List: Fix grid behaviour when expandable nested list closed

[margaree]

Can be seen here in the add-button demo at the bottom of the page. Tab in, add-button should highlight, press down arrow and focus goes to bottom add button (not expected) and then can't move away from there.

With this change, _getNextFlattenedListItem only returns the nested child when it's actually possible that that should be used for next list item focus. It's possible there are risks with this I'm missing since the logic of this is a bit complex.

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