iOS: Provide correct initial value for AppState.currentState

Summary:
Attempt to fix #7919.

Currently, if the app is launched into the background and you read `AppState.currentState` too soon, you will see the value `'active'` instead of `'background'`. This is because the default value of `AppState.currentState` is hardcoded to be `'active'` and it is initialized with the actual value asynchronously.

This attempts to fix the bug by having the `RCTAppState` module provide the initial state as a module constant.

As noted in #7919, it looks like this fix was already tried and reverted with 0fb3d8de83. zjj010104, hedgerwang, nicklockwood -- can you explain why? I would very much like to get this bug fixed. Nobody has followed up on the issue I filed so I decided to just go ahead and make a PR with my best guess at a fix.

**Test plan (required)**

Built a small app as described in the repro steps for #7919 and verified that, when the app is launched into the background, `init currentState: background` is printed. Also verified that  `i
Closes https://github.com/facebook/react-native/pull/8058

Differential Revision: D3554619

fbshipit-source-id: 5d950b85e335765552bbd3cf6ed91534062e35a1

Libraries/AppState/AppState.js

@@ -83,12 +83,10 @@ class AppState extends NativeEventEmitter {
       memoryWarning: new Map(),
     };
 
-    // 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.
-    this.currentState = 'active';
-    
+    // TODO: Remove the 'active' fallback after `initialAppState` is exported by
+    // the Android implementation.
+    this.currentState = RCTAppState.initialAppState || 'active';
+
     // 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
@@ -99,7 +97,7 @@ class AppState extends NativeEventEmitter {
         this.currentState = appStateData.app_state;
       }
     );
-    
+
     // TODO: see above - this request just populates the value of `currentState`
     // when the module is first initialized. Would be better to get rid of the prop
     // and expose `getCurrentAppState` method directly.
@@ -111,7 +109,7 @@ class AppState extends NativeEventEmitter {
     );
   }
 
-  /**   
+  /**
    * Add a handler to AppState changes by listening to the `change` event type
    * and providing the handler
    *

React/Modules/RCTAppState.m

@@ -46,6 +46,11 @@ RCT_EXPORT_MODULE()
   return dispatch_get_main_queue();
 }
 
+- (NSDictionary *)constantsToExport
+{
+  return @{@"initialAppState": RCTCurrentAppBackgroundState()};
+}
+
 #pragma mark - Lifecycle
 
 - (NSArray<NSString *> *)supportedEvents