Updating edge scrolling logic during drag and drop

Activating scroll to left when the either the visualCenter or the touchX
enters the left screen first or right screen.
Previously only the touch point was being considered to activating the scroll
zone. But visual center is used everywhere else for actual drop location. So
depending on the touch point relative to the picked icon, the touch zone could
seem smaller or larger.
Using both points (touch and visualCenter) will increase the scroll zone area
with minimal effect on the drop location calculation.

Bug: 31946075
Change-Id: I4eb5146c0f0a05e8d024313d67f8e040e0b29dec