Increase defensiveness of Incoming Header placement

This change removes the assumption that Silent notifications won't be
out of place in the ranking order. Technically, silent notifs will
never HUN, and so they should always be sorted to the bottom, but
we've seen situations where that is not the case.

Bug: 157764732
Test: manual, atest
Change-Id: Ib90f704c3b3fa290c87f8a45888424ed55d61d52