PushNotificationIOS.requestPermissions won't resolve if no event listeners are attached

Summary:
Resolves #13012

RCTPushNotificationManager uses startObserving to register for RCTRegisterUserNotificationSettings. According to the docs, the startObserving method won't be called until somebody subscribes to NotificationManagerIOS.

This means there is a scenario when the developer can call requestPermissions without subscribing to notifications first, but since RCTPushNotificationManager relies on NSNotificationCenter subscribtion, the result will never be returned.

When requesting permissions  the promise will resolve:
`PushNotificationIOS.requestPermissions().then(console.log);` without the need for calling `PushNotificationIOS.addEventListener()` first.
Closes https://github.com/facebook/react-native/pull/13263

Differential Revision: D4851767

Pulled By: javache

fbshipit-source-id: 2be8621e072ae1086014594bc986ca5590b5eb61

Libraries/PushNotificationIOS/RCTPushNotificationManager.m

@@ -142,6 +142,15 @@ RCT_EXPORT_MODULE()
                                            selector:@selector(handleRemoteNotificationReceived:)
                                                name:RCTRemoteNotificationReceived
                                              object:nil];
+  [self startObservingRegistrationNotifications];
+}
+
+- (void)startObservingRegistrationNotifications
+{
+  [[NSNotificationCenter defaultCenter] addObserver:self
+                                           selector:@selector(handleRegisterUserNotificationSettings:)
+                                               name:RCTRegisterUserNotificationSettings
+                                             object:nil];
   [[NSNotificationCenter defaultCenter] addObserver:self
                                            selector:@selector(handleRemoteNotificationsRegistered:)
                                                name:RCTRemoteNotificationsRegistered
@@ -150,10 +159,6 @@ RCT_EXPORT_MODULE()
                                            selector:@selector(handleRemoteNotificationRegistrationError:)
                                                name:RCTErrorRemoteNotificationRegistrationFailed
                                              object:nil];
-  [[NSNotificationCenter defaultCenter] addObserver:self
-                                           selector:@selector(handleRegisterUserNotificationSettings:)
-                                               name:RCTRegisterUserNotificationSettings
-                                             object:nil];
 }
 
 - (void)stopObserving
@@ -319,6 +324,7 @@ RCT_EXPORT_METHOD(requestPermissions:(NSDictionary *)permissions
     return;
   }
 
+  [self startObservingRegistrationNotifications];
   _requestPermissionsResolveBlock = resolve;
 
   UIUserNotificationType types = UIUserNotificationTypeNone;