As of https://github.com/flutter/engine/pull/47727 the web engine is able to find a default widget to focus on when a new route it pushed. The mobile engine already did that for some time. So `autofocus` is no longer necessary.
Adds a use-case screen for `RadioListTile`, similar to the `CheckBoxListTile`. This screen can help test scenarios such as the one reported in https://github.com/flutter/flutter/issues/126805.
Mostly tweaks for better focus management, namely:
* Use `autofocus` throughout so the a11y focus is transferred to a logical place when overlaid content pops up (screen transitions, dialogs).
* Consolidate "enabled" and "disabled" widgets into the same screen. Otherwise, when only a disabled widget is shown, there's nothing to focus on and the screen reader is lost.
