Drop simultaneous pointer events when tracking velocity.

They're not really simultaneous, but within 1ms of one
another, which is too close for getEventTime() to
distinguish; these events were causing the velocity
computation to become (understandably) confused and
resulting in failed panel flings.

Bug: 10246454
Change-Id: I3829192cdf9ad2a222ec8deca031d772d58de0da