Remove sharedApplication use in SafeAreaProxy (facebook#49612)

Summary:
Pull Request resolved: facebook#49612

`[UIApplication sharedApplication]` is not allowed in Mac Catalyst. This #ifdef's it out which is a common pattern elsewhere.

Changelog: [Internal]

Reviewed By: shwanton

Differential Revision: D69971189

fbshipit-source-id: 89dbc0a02ed2a06936f910fbfc13e1fb91972540

Author: sbuggay

packages/react-native/React/Base/UIKitProxies/RCTWindowSafeAreaProxy.mm

@@ -53,9 +53,11 @@ - (UIEdgeInsets)currentSafeAreaInsets
 
   // Fallback in case [startObservingSafeArea startObservingSafeArea] was not called.
   __block UIEdgeInsets insets;
+#if !TARGET_OS_MACCATALYST
   RCTUnsafeExecuteOnMainQueueSync(^{
     insets = [UIApplication sharedApplication].delegate.window.safeAreaInsets;
   });
+#endif
   return insets;
 }