[NUI] synchronize View.ExcludeLayouting and FlexLayout.PositionType (#2254)

[NUI] synchronize View.ExcludeLayouting and FlexLayout.PositionType (#2254)

Simple Test
```cs
//Default value test
bool Default_ExcludeLayouting = view.ExcludeLayouting; // False
FlexLayout.PositionType Default_FlexPositionType = FlexLayout.GetFlexPositionType(view); //Relative

// FlexPositionType test depending on ExcludeLayouting
view.ExcludeLayouting = true;
FlexLayout.PositionType FlexPositionType = FlexLayout.GetFlexPositionType(view); //Absolute

// ExcludeLayouting test depending on FlexLayout.SetFlexPositionType
FlexLayout.SetFlexPositionType(view, FlexLayout.PositionType.Relative);
bool ExcludeLayouting = view.ExcludeLayouting; // False;
```