[NUI] Unified native singleton class instance getter

[NUI] Unified native singleton class instance getter

There was 2 way to get sigleton class - `Get()` method and `Instance` proeprty.

After merge #5683 PR, some singleton class' `Get()` method register registry at that moment.
If someone call `Instance` after `Get()` call, the singleton class become GC, due to unknown reason.

To avoid this cases, let we make `Get()` method also use `Instance` property internally, and deprecate
that method.

For watch the trends after change the singletone pattern, let we just print error log if some
NUI developer implement wrong way in future.

+

Furthermore, there was some issue when we get Instance and call Dispose().
This is strance behavior. So let we just print error log and don't allow Dispose it.

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