Access AppState constants on main thread

Summary:
iOS 11 now requires this to be accessed on the main thread.
I think this is causing tests to fail, as the Main Thread Checker is turned on there.

Reviewed By: javache

Differential Revision: D5826570

fbshipit-source-id: 6acb7174c756a2ad657602f14c666f71b1ee8fe9

React/Modules/RCTAppState.m

@@ -41,9 +41,7 @@ RCT_EXPORT_MODULE()
 
 + (BOOL)requiresMainQueueSetup
 {
-  // UIApplication.applicationState seems reasonably safe to access from
-  // a background thread.
-  return NO;
+  return YES;
 }
 
 - (dispatch_queue_t)methodQueue