feat(iOS) remove deprecated [UIScreen mainScreen] references (#41388)

Summary:
The goal for this PR is to further remove references for `[UIScreen mainScreen]` and migrate them to use trait collections. This helps out of tree platforms like visionOS (where the `UIScreen` is not available).

bypass-github-export-checks

## Changelog:

[INTERNAL] [CHANGED] - use currentTraitCollection for FBSnapshotTestController.m
[IOS] [CHANGED] - use key window width to assign the correct width for RCTDevLoadingView

Pull Request resolved: #41388

Test Plan:
– Check if tests passes
- Check if `RCTDevLoadingView` shows up correctly.

Screenshot:
![CleanShot 2023-11-09 at 13 48 48@2x](https://github.com/facebook/react-native/assets/52801365/4c91399e-f70a-4e78-8288-bc7b8377c980)

Reviewed By: javache

Differential Revision: D51156230

Pulled By: cipolleschi

fbshipit-source-id: bbe711e0281046a082fd1680b55e2d117915ad00

packages/react-native/React/CoreModules/RCTDevLoadingView.mm

@@ -114,13 +114,11 @@ - (void)showMessage:(NSString *)message color:(UIColor *)color backgroundColor:(
   dispatch_async(dispatch_get_main_queue(), ^{
     self->_showDate = [NSDate date];
     if (!self->_window && !RCTRunningInTestEnvironment()) {
-      CGSize screenSize = [UIScreen mainScreen].bounds.size;
-
       UIWindow *window = RCTSharedApplication().keyWindow;
-      self->_window =
-          [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, screenSize.width, window.safeAreaInsets.top + 10)];
-      self->_label =
-          [[UILabel alloc] initWithFrame:CGRectMake(0, window.safeAreaInsets.top - 10, screenSize.width, 20)];
+      CGFloat windowWidth = window.bounds.size.width;
+
+      self->_window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, windowWidth, window.safeAreaInsets.top + 10)];
+      self->_label = [[UILabel alloc] initWithFrame:CGRectMake(0, window.safeAreaInsets.top - 10, windowWidth, 20)];
       [self->_window addSubview:self->_label];
 
       self->_window.windowLevel = UIWindowLevelStatusBar + 1;

packages/rn-tester/RCTTest/FBSnapshotTestCase/FBSnapshotTestController.m

@@ -238,8 +238,10 @@ - (NSString *)_fileNameForSelector:(SEL)selector
   if (0 < identifier.length) {
     fileName = [fileName stringByAppendingFormat:@"_%@", identifier];
   }
-  if ([[UIScreen mainScreen] scale] > 1.0) {
-    fileName = [fileName stringByAppendingFormat:@"@%.fx", [[UIScreen mainScreen] scale]];
+
+  UITraitCollection *currentTraitCollection = [UITraitCollection currentTraitCollection];
+  if (currentTraitCollection.displayScale > 1.0) {
+    fileName = [fileName stringByAppendingFormat:@"@%.fx", currentTraitCollection.displayScale];
   }
   fileName = [fileName stringByAppendingPathExtension:@"png"];
   return fileName;