Summary:
NavigationCardStack is a custom component, and its API should be explicit, not
too generic..
In NavigationCardStack, the prop `renderOverlay` is actually used to render
the NavigationHeader, and we uses absolute position to build the layout for
the header and the body.
One of the problem with using absolute postion and fixed height to build the
layout that contains the header is that the header can't have variant height
easily.
Ideally, if the layout for the header used flex-box, we'd ve able to be more
adaptive to deal with the header that has variant height.
That said, let's rename `renderOverlay` to `renderHeader`, then build the
proper layout that explicitly works better with the header.
If we to need to support overlay in navigation, we may consider add
`renderOverlay` later, if it's really necessary.
Reviewed By: ericvicenti
Differential Revision: D3670224
fbshipit-source-id: ff04acfe9dc995cb57117b3fd9b07d5f97b9c6ee
ca8531105e