st/scroll-view: Hide scroll bar from layout when its hidden
We hide scrollbars with POLICY_AUTOMATIC when the content of the scroll
view fits completely inside the view without having to scroll. In this case
it seems like a good idea to give the content the full available size
without subtracting scroll bars from that size.
So subtract the scroll bar from the size we give to the child only when
the scroll bar is actually visible, when it's invisible subtract 0 instead.