Add a snapshot only if the Modal is visible (#42619)

Summary:
Pull Request resolved: #42619

This is a workaround to prepare for the next diff in the stack and make sure that the modal works correctly

## Changelog
[iOS][Changed] - Add the for the dismissal snapshot only when we need it.

Reviewed By: sammy-SC

Differential Revision: D53003657

fbshipit-source-id: 6d6cc85946b1beb8e784e08a650d1247cf780228

packages/react-native/React/Fabric/Mounting/ComponentViews/Modal/RCTModalHostViewComponentView.mm

@@ -165,7 +165,10 @@ - (void)ensurePresentedOnlyIfNeeded
     // To animate dismissal of view controller, snapshot of
     // view hierarchy needs to be added to the UIViewController.
     UIView *snapshot = _modalContentsSnapshot;
-    [self.viewController.view addSubview:snapshot];
+
+    if (_shouldPresent) {
+      [self.viewController.view addSubview:snapshot];
+    }
 
     [self dismissViewController:self.viewController
                        animated:_shouldAnimatePresentation
@@ -194,7 +197,7 @@ - (void)ensurePresentedOnlyIfNeeded
 - (void)mountingTransactionWillMount:(const MountingTransaction &)transaction
                 withSurfaceTelemetry:(const facebook::react::SurfaceTelemetry &)surfaceTelemetry
 {
-  _modalContentsSnapshot = [self.viewController.view snapshotViewAfterScreenUpdates:NO];
+  _modalContentsSnapshot = [self.viewController.view snapshotViewAfterScreenUpdates:YES];
 }
 
 #pragma mark - UIView methods