[Fabric] Add flex: 1 to Offscreen view container (facebook#22019)

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,
+      },
     };
   }
 }