Allow the user to collapse the notification panel on quick settings

Previously we were immediately disallowing to intercept when the
scrolling started on the header. This is wrong, since the user
could also swipe upwards. We're now doing this only once the
touchslop has been crossed.

Fixes: 159679534
Test: start collapsing on the QS header, observe that it works. Other interactions are unchanged
Change-Id: Iee86b2a8a473664846a6b119bf9110b5a751a2a8