Avoid race condition for AppState.currentState

Summary:
When the AppState module is initialized, it subscribes to the `appStateDidChange` event and sends an async native method call to the AppState native module. There is a small race condition window where the native module can read the current app state as `uninitialized` before calling the JavaScript callback, and then be interrupted by the underlying mechanism to trigger the `appStateDidChange` event. If the `appStateDidChange` event is processed before the JavaScript callback, the resulting value of `AppState.currentState` will be invalid.

Fixes Microsoft/react-native-windows#1300

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!)

I simulated (over-exaggerated) the race condition by injecting "thread sleep" calls in the native method call and the native mechanism for updating the app state.

I then ran the AppStateExample in the RNTester and found that the current app state was set to `uninitialized`, as opposed to the expected value of `active`.

Once I made this JavaScript change, the over-exaggerated race condition no longer resulted in an invalid app state.
Closes https://github.com/facebook/react-native/pull/15499

Differential Revision: D5660620

Pulled By: hramos

fbshipit-source-id: 47c0dca75d37f677191c48f2148a72edd9cdd0e2

Libraries/AppState/AppState.js

@@ -102,6 +102,8 @@ class AppState extends NativeEventEmitter {
     // the Android implementation.
     this.currentState = RCTAppState.initialAppState || 'active';
 
+    let eventUpdated = false;
+
     // TODO: this is a terrible solution - in order to ensure `currentState` prop
     // is up to date, we have to register an observer that updates it whenever
     // the state changes, even if nobody cares. We should just deprecate the
@@ -109,6 +111,7 @@ class AppState extends NativeEventEmitter {
     this.addListener(
       'appStateDidChange',
       (appStateData) => {
+        eventUpdated = true;
         this.currentState = appStateData.app_state;
       }
     );
@@ -118,7 +121,9 @@ class AppState extends NativeEventEmitter {
     // and expose `getCurrentAppState` method directly.
     RCTAppState.getCurrentAppState(
       (appStateData) => {
-        this.currentState = appStateData.app_state;
+        if (!eventUpdated) {
+          this.currentState = appStateData.app_state;
+        }
       },
       logError
     );