Fix initial scenes rendering.

Summary:The initial layout used to render scenes does not contain the actual
width and height measured and causes the issue as described at
https://github.com/ericvicenti/navigation-rfc/issues/61

The fix is to update the layout and re-render scenes once layout
is modified. Also scenes renderer should also consider the case that
when the layout is not measured yet.

Reviewed By: ericvicenti

Differential Revision: D3162143

fb-gh-sync-id: 197574329d3849cad2a21e07e1bd5e800f74c3ea
fbshipit-source-id: 197574329d3849cad2a21e07e1bd5e800f74c3ea

Libraries/NavigationExperimental/NavigationPropTypes.js

@@ -52,6 +52,7 @@ const layout = PropTypes.shape({
   height: animatedValue,
   initHeight: PropTypes.number.isRequired,
   initWidth: PropTypes.number.isRequired,
+  isMeasured: PropTypes.bool.isRequired,
   width: animatedValue,
 });