SystemUI: improve one finger notification expansion

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

Conflicts:
	packages/SystemUI/src/com/android/systemui/ExpandHelper.java