Adjust touch position for handles using window coordinate.

Touch position is adjusted in HandleView#onTouchEvent to snap to
ideal offset.
Previously, screen coordinate was used for input, but window
coordinate is used for the ideal offset. As a result, window adjusted touch
position will be adjusted to improper position and a handle being
dragged will get moved to unintended line.

Bug: 22308836

Change-Id: I4a5a3cc695a874c85aa341f06ffcba8b7f5e318c