SystemUI: improve one finger notification expansion

Fixes JIRA bug BACON-70.

Sometimes the quick settings ribbon would eat the initial ACTION_DOWN
setting mInitialTouchY in the expand helper, but we are clearly not in a
notification that we can expand here. So only set the initial y point if
we are inside the notification scrollview.

Change-Id: I628553ac15cc6b4ee3c682fe46bd1d96c6bc154d
Signed-off-by: Roman Birg <roman@cyngn.com>