fix: over reporting trait changes (facebook#39439)

Summary:
Closes facebook#35972
Closes facebook#36713

This PR addresses a couple of issues with `useColorScheme` and the `Appearance` API.

- facebook#38214 introduced a regression. Using to `RCTExecuteOnMainQueue` was a mistake as we need this to happen synchronously to return the result. Doing it async causes the `traitCollection` to remain uninitialized.
- The `useColorScheme` hook is updating when the app is in the background on iOS and the OS is taking the snapshots for the app switcher. This causes a flash when returning to the app as the correct color is set again. Here, we can check for the app state in `traitCollectionDidChange` and not send these events when in the background.
- Removed a line that was left over after some OS version checks were removed when support for iOS 12 was dropped.

## Changelog:

[IOS] [FIXED] - Don't send the `RCTUserInterfaceStyleDidChangeNotification` when the app is in the background.

Pull Request resolved: facebook#39439

Test Plan: Tested on `rn-tester`, logged the changes whenever `useColorScheme` updates. It no longer happens when the app is in the background. The returned interface style on the initial render is always correct now.

Reviewed By: NickGerleman

Differential Revision: D49454281

Pulled By: javache

fbshipit-source-id: 87e24158a49c50608c79e73fb484442f5aad36a6

packages/react-native/React/Base/RCTRootView.m

@@ -368,6 +368,9 @@ - (void)contentViewInvalidated
 - (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection
 {
   [super traitCollectionDidChange:previousTraitCollection];
+  if (RCTSharedApplication().applicationState == UIApplicationStateBackground) {
+    return;
+  }
 
   [[NSNotificationCenter defaultCenter]
       postNotificationName:RCTUserInterfaceStyleDidChangeNotification

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

@@ -17,4 +17,5 @@ RCT_EXTERN NSString *RCTCurrentOverrideAppearancePreference();
 RCT_EXTERN NSString *RCTColorSchemePreference(UITraitCollection *traitCollection);
 
 @interface RCTAppearance : RCTEventEmitter <RCTBridgeModule>
+- (instancetype)init;
 @end

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

@@ -57,11 +57,7 @@ void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride)
     return RCTAppearanceColorSchemeLight;
   }
 
-  traitCollection = traitCollection ?: [UITraitCollection currentTraitCollection];
   return appearances[@(traitCollection.userInterfaceStyle)] ?: RCTAppearanceColorSchemeLight;
-
-  // Default to light on older OS version - same behavior as Android.
-  return RCTAppearanceColorSchemeLight;
 }
 
 @interface RCTAppearance () <NativeAppearanceSpec>
@@ -71,6 +67,19 @@ @implementation RCTAppearance {
   NSString *_currentColorScheme;
 }
 
+- (instancetype)init
+{
+  if ((self = [super init])) {
+    UITraitCollection *traitCollection = RCTSharedApplication().delegate.window.traitCollection;
+    _currentColorScheme = RCTColorSchemePreference(traitCollection);
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(appearanceChanged:)
+                                                 name:RCTUserInterfaceStyleDidChangeNotification
+                                               object:nil];
+  }
+  return self;
+}
+
 RCT_EXPORT_MODULE(Appearance)
 
 + (BOOL)requiresMainQueueSetup
@@ -100,9 +109,6 @@ - (dispatch_queue_t)methodQueue
 
 RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSString *, getColorScheme)
 {
-  if (_currentColorScheme == nil) {
-    _currentColorScheme = RCTColorSchemePreference(nil);
-  }
   return _currentColorScheme;
 }
 
@@ -129,14 +135,15 @@ - (void)appearanceChanged:(NSNotification *)notification
 
 - (void)startObserving
 {
-  [[NSNotificationCenter defaultCenter] addObserver:self
-                                           selector:@selector(appearanceChanged:)
-                                               name:RCTUserInterfaceStyleDidChangeNotification
-                                             object:nil];
 }
 
 - (void)stopObserving
 {
+}
+
+- (void)invalidate
+{
+  [super invalidate];
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }