Add RCTOverrideAppearancePreference to iOS Appearance module

Summary:
In order to enable more fine-grained control of theming in brownfield apps, this adds RCTOverrideAppearancePreference to RCTAppearance.

## Changelog:
[iOS] [Added] - Adds RCTOverrideAppearancePreference to the iOS Appearance module

Reviewed By: sammy-SC

Differential Revision: D19187657

fbshipit-source-id: 52783c497d32d36af2523fce6f040d6cfb5aac3c

Author: Zack Argyle

React/CoreModules/RCTAppearance.h

@@ -11,6 +11,7 @@
 #import <React/RCTEventEmitter.h>
 
 RCT_EXTERN void RCTEnableAppearancePreference(BOOL enabled);
+RCT_EXTERN void RCTOverrideAppearancePreference(NSString *const);
 
 @interface RCTAppearance : RCTEventEmitter <RCTBridgeModule>
 @end

React/CoreModules/RCTAppearance.mm

@@ -23,13 +23,22 @@ void RCTEnableAppearancePreference(BOOL enabled) {
   sAppearancePreferenceEnabled = enabled;
 }
 
+static NSString *sColorSchemeOverride = nil;
+void RCTOverrideAppearancePreference(NSString *const colorSchemeOverride) {
+  sColorSchemeOverride = colorSchemeOverride;
+}
+
 static NSString *RCTColorSchemePreference(UITraitCollection *traitCollection)
 {
 #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && defined(__IPHONE_13_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_13_0
   if (@available(iOS 13.0, *)) {
     static NSDictionary *appearances;
     static dispatch_once_t onceToken;
 
+    if (sColorSchemeOverride) {
+      return sColorSchemeOverride;
+    }
+
     dispatch_once(&onceToken, ^{
       appearances = @{
                       @(UIUserInterfaceStyleLight): RCTAppearanceColorSchemeLight,