[NUI] Clean BackgroundImage setter logic

[NUI] Clean BackgroundImage setter logic

Usually we load image load asynchronously as default.

(The only one except case is lottie.
But most of cases we don't need to consider
set BackgroundImage as Lottie file.)

So, for usual cases, we don't need to apply the informations whether
we need to load image synchronously or not.

But previous implementation set `BackgroundImageSynchronousLoading` forcibly.

It might required `BackgroundImage` getter, what we don't need to know.

And also, it doesn't skip this getter/setter logic when the value is same as previous.

To avoid this useless visual creation, let we consider for backgroundImageSynchronousLoading
property works well and works faster for usual cases, who don't set this property.

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