enable RTL layout by default for all apps

Summary:
We're now enabling RTL layout by default assuming the app has the proper localized assets. Previously, this was disabled by default to minimize surprise. Apps that don't want this RTL support can still manually disable them by using `RCTI18nUtil.allowRTL(false)` (iOS) or `I18nUtil.allowRTL(false)` (android) in the apps start-up logic, as outlined in the blog post: http://facebook.github.io/react-native/blog/2016/08/19/right-to-left-support-for-react-native-apps.html

iOS Util function: https://github.com/facebook/react-native/blob/f0fb228ec76ed49e6ed6d786d888e8113b8959a2/React/Modules/RCTI18nUtil.m#L53
Android Util function: https://github.com/facebook/react-native/blob/380830e4aa212fb934d3fe82d971cb8a316671f8/ReactAndroid/src/main/java/com/facebook/react/modules/i18nmanager/I18nUtil.java#L63

Differential Revision: D3825054

fbshipit-source-id: 88b355fef9e3847939a414f80d2285979e27af08

React/Modules/RCTI18nUtil.m

@@ -42,12 +42,15 @@ - (BOOL)isRTL
 /**
  * Should be used very early during app start up
  * Before the bridge is initialized
+ * @return whether the app allows RTL layout, default is true
  */
 - (BOOL)isRTLAllowed
 {
-  BOOL rtlStatus = [[NSUserDefaults standardUserDefaults]
-                            boolForKey:@"RCTI18nUtil_allowRTL"];
-  return rtlStatus;
+  NSNumber *value = [[NSUserDefaults standardUserDefaults] objectForKey:@"RCTI18nUtil_allowRTL"];
+  if (value == nil) {
+    return YES;
+  }
+  return [value boolValue];
 }
 
 - (void)allowRTL:(BOOL)rtlStatus

ReactAndroid/src/main/java/com/facebook/react/modules/i18nmanager/I18nUtil.java

@@ -32,7 +32,7 @@ private I18nUtil() {
   }
 
   public static I18nUtil getInstance() {
-    if(sharedI18nUtilInstance == null) {
+    if (sharedI18nUtilInstance == null) {
       sharedI18nUtilInstance = new I18nUtil();
     }
     return sharedI18nUtilInstance;
@@ -55,9 +55,10 @@ public boolean isRTL(Context context) {
   /**
    * Should be used very early during app start up
    * Before the bridge is initialized
+   * @return whether the app allows RTL layout, default is true
    */
   private boolean isRTLAllowed(Context context) {
-    return isPrefSet(context, KEY_FOR_PREFS_ALLOWRTL, false);
+    return isPrefSet(context, KEY_FOR_PREFS_ALLOWRTL, true);
   }
 
   public void allowRTL(Context context, boolean allowRTL) {