- Move `setState` to only be invoked when guarded by `widget._allowMultiplePanelsOpen`, fixing the case for `ExpansionPanelList`
- Remove setting `_currentOpenPanel` to `widget.initialOpenPanelValue` in `didUpdateWidget`, since this should only occur on `initState` and not every time the widget is updated. This fixes the problem for `ExpansionPanelList.radio`
- Added a `didUpdateWidget` condition for when `ExpansionPanelList` changes into `ExpansionPanelList.radio` to open the panel at `widget.initialOpenPanelValue`
- Added test cases for regression, expansionCallback cases, and `didUpdateWidget` transitions between `ExpansionPanelList` and `ExpansionPanelList.radio`
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