SystemUI: Hide notifications when quickly expanding QS

Expand/collapse animation with the 2 finger swipe/swipe down from
right side to expand QS gesture [1] was pretty ugly with all the
notifications being dragged along. Let's hide the notifications
stack entirely when quickly expanding QS, for a nicer looking
animation similar to the one in lockscreen.

[1]: https://github.com/pa-gr/android_frameworks_base/commit/d3622399728c819c45c5435ddd0b1e359207f962

Change-Id: Id2dbc707f9da543d47d76ae5525e827b400a4e7b