Summary:
Decreased the distance threshold so that the user doesn't need to swipe a large distance to
set the pan responder.
Also, set a threshold for the velocity so that a simple flick can be detected.
Reviewed By: hedgerwang
Differential Revision: D3540354
fbshipit-source-id: 251a4f14dc014bc32b3a83fa8de419f86ca40b84