Fix up tests after Shell runs

[PureWeen]

Description of Change

• Shell sets the background of the decorview to a specialized "splitDrawable" that was breaking the UnitTests after a run. The background now just gets reset to pre-shell state so that tests remain runnable
• The NavigationRootManager was being added to the WrapperView as a WeakReference which means that sometimes it would get cleaned up by the GC since that's the only reference. The NavigationRootManager needs to exist as a strong reference inside the WrappedServiceProvider
  • This fixed an issue where various tests would occasionally fail with "NavigationRootManager" not found
• Fixed up some fragments inside shell so they create a new scoped MauiContext with the correct inflater/childsupportmanager
• Fixed Editor test on WinUI so that it adds the editor to the VisualTree. This is required in order for the editor field to measure at all

Testing

• verify tests all run properly in visual runner and that you can run and re-run them
• verify Android Shell still works