[Notifications KCM] Fixup current item syncing logic

Instead of relying on currentIndex and some massively complicated logic to track when the index didn't change
but the item behind that index, just use currentItem.
The updateCurrentIndex being called during model changes massively confused ListView leading to inconsistent view state.

BUG: 410842
FIXED-IN: 5.17.5

Differential Revision: https://phabricator.kde.org/D26425