Set UNSELECTED_POSITION from SelectionHandler.unselectedCellView()

[mgodf]

SelectionHandler tracks which row and column is currently selected with mSelectedRowPosition mSelectedColumnPosition variables. When clearing the selection, these should also be set to UNSELECTED_POSITION. If this is not done, when the previously selected cell is scrolled out of view and then back, the previous selection will be restored when the viewholder is bound again