Fix a race condition in AppState that prevents listeners from being notified

Summary:
If someone has setup a subscription on AppState and we correct AppState via getCurrentAppState call, we need to notify all the subscribers of AppState.
1 ) Initial AppState.currentState = 'active'
2-start) Subscribe to AppState Changes
3-start) Fetch Current AppState
4 ) App Code subscribes to AppState module
5 ) App becomes backgrounded
2-finish) AppState listeners are setup (missing background event)
3-finish) AppState.currentState updated to background

At this point the subscription setup in 4) will never be called with the change.

AppState should always call subscribers on change

This is very difficult to formally test since it's due to a race condition. We've seen this condition via bug reports but have had no local repro.

[GENERAL][BUGFIX][AppState] - Fix a race condition that could prevent AppState subscription change listener from firing on initial launch
Closes https://github.com/facebook/react-native/pull/18236

Differential Revision: D7823370

Pulled By: hramos

fbshipit-source-id: 99b174df70262ceaf9da141d005131facd624594

Libraries/AppState/AppState.js

@@ -60,8 +60,10 @@ class AppState extends NativeEventEmitter {
     // prop and expose `getCurrentAppState` method directly.
     RCTAppState.getCurrentAppState(
       (appStateData) => {
-        if (!eventUpdated) {
+        // It's possible that the state will have changed here & listeners need to be notified
+        if (!eventUpdated && this.currentState !== appStateData.app_state) {
           this.currentState = appStateData.app_state;
+          this.emit('appStateDidChange', appStateData);
         }
       },
       logError