[NUI] Allow to use null rootView for FrameUpdateCallback + Let we use null rootView...

[NUI] Allow to use null rootView for FrameUpdateCallback + Let we use null rootView if we use default API

Let we use some global view controllable frameUpdateCallback when
user skip to add RootView information.

Previously, we detach the frameUpdateCallback automatically if rootView was disposed.
But now, we will never remove callback until user call RemoveFrameUpdateCallback.

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