Use 'background' app state when app is foregrounded from notif

Summary:Changing app state back to 'background' for UIApplicationWillEnterForegroundNotification.

On iOS we use the 'background' app state to determine whether a notification was tapped to foreground the app vs. received while app was already active. The PR facebook#6379 changed RCTAppState so that it returned 'active' when app was being foregrounded from a notification; this changes it back to 'background' so that we can distinguish between the two cases again

Reviewed By: hedgerwang

Differential Revision: D3078746

fb-gh-sync-id: 8b5e9118a7e14f15871bfb68e9f85d20108b1faf
shipit-source-id: 8b5e9118a7e14f15871bfb68e9f85d20108b1faf

React/Modules/RCTAppState.m

@@ -88,7 +88,7 @@ - (void)handleAppStateDidChange:(NSNotification *)notification
   if ([notification.name isEqualToString:UIApplicationWillResignActiveNotification]) {
     newState = @"inactive";
   } else if ([notification.name isEqualToString:UIApplicationWillEnterForegroundNotification]) {
-    newState = @"active";
+    newState = @"background";
   } else {
     newState = RCTCurrentAppBackgroundState();
   }