[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.
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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>