[NUI] change size specification in changed callback of bindable property (#2516)

[NUI] change size specification in changed callback of bindable property (#2516)

If we change size2D by using Size2DProperty, size specification is not
changed. this patch will fix that issue.

Here is sample code:
```cs
View view = new View();
view.Size2D = new Size2D(100, 50);
Tizen.Log.Error("NUI", $"WidthSpecification : {view.WidthSpecification}, HeightSpecification : {view.HeightSpecification}");

View view2 = new View();
view2.SetValue(View.Size2DProperty, new Size2D(100, 50));
Tizen.Log.Error("NUI", $"WidthSpecification : {view2.WidthSpecification}, HeightSpecification : {view2.HeightSpecification}");
```

Before:
```
WidthSpecification : 100, HeightSpecification : 50
WidthSpecification : -2, HeightSpecification : -2
```

After:
```
WidthSpecification : 100, HeightSpecification : 50
WidthSpecification : 100, HeightSpecification : 50
```

Co-authored-by: Jiyun Yang <ji.yang@samsung.com>