[Inspector V2][Tech debt] `FlexLayoutExplorer` refactors

After https://github.com/flutter/devtools/pull/8202, there are a few refactors that should be made to the `FlexLayoutExplorerWidget` (making a tracking issue for this work instead of trying to do it all in https://github.com/flutter/devtools/pull/8202 since that PR is already huge).

- [ ] Because `BoxLayoutExplorerWidget` is no longer a subclass of `LayoutExplorerWidget`, `FlexLayoutExplorerWidget` is now its only subclass. Therefore, its logic can be combined with `LayoutExplorerWidget`.
- [ ] Now that the `FlexLayoutExplorerWidget` is in a tabbed view, the animation is never visible. Therefore the animation should be removed.
- [ ] A lot of the logic in `LayoutExplorerWidget` has been pulled out into the `InspectorController` (e.g., `_loadLayoutPropertiesForNode`.) Instead of having this logic in two places, we should refactor the `FlexLayoutExplorer` to accept the layout properties etc. as arguments instead of computing them itself.
- [ ] Would also be nice to rename `BoxLayoutExplorerWidget` to `BoxLayoutExplorer` and `FlexLayoutExplorerWidget` to `FlexLayoutExplorer`.

Start with FlexLayoutExplorerWidget, LayoutExplorerWidget, BoxLayoutExplorerWidget, and InspectorController, comparing the related changes from PR 8202. Trace the duplicated layout-property logic and confirm whether the animation is still reachable in the tabbed view. Done means the widgets are consolidated or renamed as appropriate, unused animation is removed, and layout properties are supplied consistently rather than recomputed.

Scritto dal modello di indicizzazione a partire dal testo della issue.