Disables dismissing notification on leftward swipe.

This change modifies SwipeHelper to pass the direction of a swipe to the
provided callback when determining whether the swipe represents a valid
dismiss gesture, and modifies the callback defined by
NotificationStackScrollLayout to allow dismiss gestures only to the
right.

Test: manually
Change-Id: I7ff1e8f92e7da9ed36520195d823f9a62f9a64ba