computeScrollVectorForPosition
Should calculate the vector that points to the direction where the target position can be found.
This method is used by the LinearSmoothScroller to initiate a scroll towards the target position.
The magnitude of the vector is not important. It is always normalized before being used by the LinearSmoothScroller.
LayoutManager should not check whether the position exists in the adapter or not.
Return
the scroll vector for a given position.
Parameters
targetPosition
the target position to which the returned vector should point