[IOS] Notification observer remove timing issue

[yenoss]

Description of the issue

The screenshot callback plugin has a dispose method.

But the observer will be not releasing when the dispose method.
This is because the observer is released when deinit is called from ios code.
This logic will be different from what the plugin's user expects.

So I think this should be fixed.

Thanks.

[songyiYu]

@minsub0922