Fixes Appearance getColorScheme sync method UI thread issue (#46344)

Summary:
If the module sets the method queue to the main queue, we should call it on the main queue if it contains some UI operations, otherwise it may lead to some undefined behavior.

## Changelog:

[IOS] [FIXED] - Fixes the exported synchronous method not being called on the method queue when it's the main queue

Pull Request resolved: #46344

Test Plan: The sync method should be called on the main queue if the module's method queue is main queue.

Reviewed By: cipolleschi

Differential Revision: D63532525

Pulled By: javache

fbshipit-source-id: 55baaa60af96bb1355d3641174f23bccd8eb9344

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

@@ -130,7 +130,10 @@ - (dispatch_queue_t)methodQueue
 RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSString *, getColorScheme)
 {
   if (!sIsAppearancePreferenceSet) {
-    UITraitCollection *traitCollection = RCTKeyWindow().traitCollection;
+    __block UITraitCollection *traitCollection = nil;
+    RCTUnsafeExecuteOnMainQueueSync(^{
+      traitCollection = RCTKeyWindow().traitCollection;
+    });
     _currentColorScheme = RCTColorSchemePreference(traitCollection);
   }
   return _currentColorScheme;