Make notification panel delete-all animation smoother

Making the notfication delete-all animation smoother by carefully
choreographing the various parts of it. The problem with the previous
animation was that there was simply too much going on at the
same time, causing things like layout and recreating display-lists
in the middle of animations that became choppy as a result. This
approach swipes all items off quickly, then scrolls the shade up to the
top, making both sets of animations smoother as a result.

Fixes #5431207: Notification delete-all should be smoother

Change-Id: Iefe8ab5d661e05adcd10379dab85227d17904450