Fix flickering (facebook#41461)

Summary:
Pull Request resolved: facebook#41461

D51179007 updated the layout spec so that we wouldn't have the weird spacing. However, it introduced some weird flickering through the app.

Upon investigating in layout inspector, I found that the issue actually isn't the rn_fragment, it's the rootview being created without the proper specs. This updates the ReactNavigationFragment so that the root view gets the correct specs, similar to how it's done here: https://www.internalfb.com/code/fbsource/[80683c89da108bba70bfe60340f80c6613960119]/fbandroid/java/com/facebook/fbreact/navigation/ReactNavigationActivityDelegate.java?lines=109

[changelog] internal

Reviewed By: jessebwr

Differential Revision: D51267353

fbshipit-source-id: 6deb5f29e8a73d9cf38cb9445d69ab041b974ec8

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactFragment.java

@@ -30,7 +30,7 @@ public class ReactFragment extends Fragment implements PermissionAwareActivity {
   protected static final String ARG_LAUNCH_OPTIONS = "arg_launch_options";
   protected static final String ARG_FABRIC_ENABLED = "arg_fabric_enabled";
 
-  private ReactDelegate mReactDelegate;
+  protected ReactDelegate mReactDelegate;
 
   @Nullable private PermissionListener mPermissionListener;