Change stylus button press to context click in GD - API review feedback

Based on API review feedback we should use onContextClick instead of
onStylusButtonPress. This CL updates associated methods and usage
in GestureDetector.

Additionally listens for mouse right clicks and calls the
onContextClick method for these as well.

Bug: 21572278
Change-Id: I4fa01997bff7265d5d2713353a93e07417a148e5