[Fabric] Add flex: 1 to Offscreen view container

Without this style property, the layout of the children is messed up.

The goal is for the container view to have no layout at all. It should
be a completely transparent wrapper, except for when we set `display:
none` to hide its contents. On the web, the equivalent (at least in the
spec) is `display: contents`.

After some initial testing, this seems to be close enough to the desired
behavior that we can ship it. We'll try rolling it out behind a flag.

packages/react-native-renderer/src/ReactFabricHostConfig.js

@@ -453,6 +453,9 @@ export function getOffscreenContainerProps(
   } else {
     return {
       children,
+      style: {
+        flex: 1,
+      },
     };
   }
 }