Add an interface for Parent view to negotiate with child Scrollable View.

Created by: changxiangzhong

Use case: Assume a sliding-up-menu child is also a Scrollable (e.g. ListView/RecyclerView/ScrollView). A common requirement is if the child has scroll to topmost, then any scroll-down should be handled by SlidingUpPanelLayout, i.e. when the menu is on its topmost position, any scroll-down gesture would close the menu.

To achieve this, The parent SlidingUpPanelLayout needs to consult if the child has been on its topmost position. If yes, the parent would INTERCEPT the whole gesture. Otherwise the parent would simply have the child to handle it.