Clean up NotificationIconAreaController.shouldShowNotificationIcon() for new pipeline

In the new pipeline:
* rows are never GONE.
* the List<ListEntry> is hierarchical, so isTopLevelChild() is redundant.
* "low priority" and "ambient" map to "silent section" and "minimized", so use the pipeline-assigned section to perform the filtering.

Understanding the pipeline-assigned section's type required adding the "silent" section classifications to the SectionStyleProvider.

isTopLevelChild() is now unused, so it has been removed.  Yay!

Fixes: 237296171
Test: atest NotificationIconAreaControllerTest
Change-Id: I8c4b0435fdb43be2b0f99728ef8fbc8733ea1c43