[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`.