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 https://github.com/facebook/react-native/pull/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