Clear selected widget on scroll.

This changelist ensures that the selected widget is cleared when
scrolling in edit mode. This prevents unexpected rendering due to
maintaining the selected highlight when the widget leaves the viewable
space.

Test: atest CommunalEditModeViewModelTest#scrollPosition_clearsSelectedItem
Fixes: 395086108
Flag: EXEMPT bugfix
Change-Id: I6ce7e66462706bafe44bc9cadea16128187f5dae