public void stopTracking() {
            if (mTracking) {
                mBuilder.append("</DSLVStates>\n");
                flush();
                mTracking = false;
            }
        }
Return a scroll speed in pixels/millisecond. Always return a
positive number.

@param w Normalized position in scroll region (i.e. w \in [0,1]).
Small w typically means slow scrolling.
@param t Time (in milliseconds) since start of scroll (handy if you
want scroll acceleration).
@return Scroll speed at position w and time t in pixels/ms.
third_party/DragSortListView/src/com/mobeta/android/dslv/DragSortListView.java