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: I877a6d5db4ec02d24f734d3284d554da13608322
Merge "SystemUI: improve one finger notification expansion" into du44