Fixed an issue where would could be left with empty notifications

If the state between systemui and the notification manager got
out of sync, we would not immediately remove the notifications when
using clear all.

This could lead to holes in the notification shade
temporarily. While this isn't a fix, it helps avoiding these ugly
states, similarly to how we already do it when manually swiping.

This also improves that only notifications that are visible are
actually participating in the clear all animations instead of
all of them.

Test: manual, hit clear all observe animations / normal clearing
Bug: 62171447
Change-Id: I83d0f3cb0bae4bc43bf35d3f9399cebc05259680