Use native initialAppState to init app_state for AppStateIOS.

Libraries/AppStateIOS/AppStateIOS.ios.js

@@ -122,11 +122,7 @@ var AppStateIOS = {
     _eventHandlers[type].delete(handler);
   },
 
-  // TODO: getCurrentAppState callback seems to be called at a really late stage
-  // after app launch. Trying to get currentState when mounting App component
-  // will likely to have the initial value here.
-  // Initialize to 'active' instead of null.
-  currentState: ('active' : ?string),
+  currentState: (RCTAppState && RCTAppState.initialAppState : ?string),
 
 };
 

React/Modules/RCTAppState.m

@@ -96,4 +96,13 @@ RCT_EXPORT_METHOD(getCurrentAppState:(RCTResponseSenderBlock)callback
   callback(@[@{@"app_state": _lastKnownState}]);
 }
 
+#pragma mark - RCTBridgeModule
+
+- (NSDictionary *)constantsToExport
+{
+  return @{
+     @"initialAppState" : RCTCurrentAppBackgroundState()
+   };
+}
+
 @end