[NUI] Fix dispose timing error for ImageUrl

[NUI] Fix dispose timing error for ImageUrl

Since we update ImageView lazy, ImageUrl's lifecycle might not matched
what user think.

```
ImageUrl imageUrl = ~~~.GenerateUrl();
ImageView imageView = new ImageView(imageUrl.ToString());
imageUrl.Dispose(); ///< We should allow this situation.
```

To make scene, We allow to call UpdateImage(); synchoronously
if we use GeneratedUrl().

Signed-off-by: Eunki Hong <eunkiki.hong@samsung.com>