Recognizes the stylus scale gesture in ScaleGestureDetector

If a user drags with a stylus (or mouse) across the screen with
the stylus primary (or mouse secondary) button clicked, this will
initiate a scale gesture that functions similarly to the quick
scale gesture: drag below starting point zooms in, drag above
starting point zooms out.

Combines some of the logic for quick scale and button
scale due to similarities.

Bug: 19620246
Change-Id: Ic53499d07cf6cef48b467dbb41f83cea433febad