## Description
This factors out a separate `RawView` that doesn't add a `MediaQuery` or a `FocusScope`. This PR also adds a new method `WidgetsBindingObserver.didChangeViewFocus` which allows the observer to know when the `FlutterView` that has focus has changed.
It also makes the `View` widget a stateful widget that contains a `FocusScope` and ` FocusTraversalGroup` so that it can respond to changes in the focus of the view.
I've also added a new function to `FocusScopeNode` that will allow the scope node itself to be focused, without looking for descendants that could take the focus. This lets the focus be "parked" at the `FocusManager.instance.rootScope` so that nothing else appears to have focus.
## Tests
- Added tests for the new functionality.
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