Back out "fix: return the correct default trait collection"

Summary:
Original commit changeset: f069fea8918f

Original Phabricator Diff: D49186069

bypass-github-export-checks

theory: getKeyWindowTraitCollection could get run in an async block, so there's a race to set the value. currently it gets read as nil on the first attempt

Reviewed By: sammy-SC

Differential Revision: D49449392

fbshipit-source-id: 3c71403fb30fa873a49bdc754c77797ed292a3f0

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

@@ -36,15 +36,6 @@ void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride)
   return sColorSchemeOverride;
 }
 
-static UITraitCollection *getKeyWindowTraitCollection()
-{
-  __block UITraitCollection *traitCollection = nil;
-  RCTExecuteOnMainQueue(^{
-    traitCollection = RCTSharedApplication().delegate.window.traitCollection;
-  });
-  return traitCollection;
-}
-
 NSString *RCTColorSchemePreference(UITraitCollection *traitCollection)
 {
   static NSDictionary *appearances;
@@ -66,7 +57,7 @@ void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride)
     return RCTAppearanceColorSchemeLight;
   }
 
-  traitCollection = traitCollection ?: getKeyWindowTraitCollection();
+  traitCollection = traitCollection ?: [UITraitCollection currentTraitCollection];
   return appearances[@(traitCollection.userInterfaceStyle)] ?: RCTAppearanceColorSchemeLight;
 
   // Default to light on older OS version - same behavior as Android.