Fixes iOS reload through metro "r" command key (facebook#28477)

Summary:
This allows the iOS device to be reloaded through the metro command line, besides the fact that whenever packagerServerHost is called, it will only get the IP address once when debugging.

## Changelog

[iOS] [Fixed] - Fixed connection of metro reload command to iOS device
Pull Request resolved: facebook#28477

Test Plan:
- Build any react-native project in debug mode to an iOS device connected through USB
- Press the “r” key on the terminal that is running metro
- The device should now reload the project

Reviewed By: cpojer

Differential Revision: D20818462

Pulled By: TheSavior

fbshipit-source-id: 6d9792447d205223dad8fbd955518885427cbba8

React/Base/RCTBundleURLProvider.h

@@ -31,6 +31,12 @@ extern const NSUInteger kRCTBundleURLProviderDefaultPort;
  */
 - (void)resetToDefaults;
 
+/**
+ * Return the server host. If its a development build and there's no jsLocation defined,
+ * it will return the server host IP address
+ */
+- (NSString *)packagerServerHost;
+
 #if RCT_DEV
 - (BOOL)isPackagerRunning:(NSString *)host;
 #endif

React/DevSupport/RCTPackagerConnection.mm

@@ -40,7 +40,7 @@ @implementation RCTPackagerConnection {
   std::mutex _mutex; // protects all ivars
   RCTReconnectingWebSocket *_socket;
   BOOL _socketConnected;
-  NSString *_jsLocationForSocket;
+  NSString *_serverHostForSocket;
   id _bundleURLChangeObserver;
   uint32_t _nextToken;
   std::vector<Registration<RCTNotificationHandler>> _notificationRegistrations;
@@ -62,8 +62,8 @@ - (instancetype)init
 {
   if (self = [super init]) {
     _nextToken = 1; // Prevent randomly erasing a handler if you pass a bogus 0 token
-    _jsLocationForSocket = [RCTBundleURLProvider sharedSettings].jsLocation;
-    _socket = socketForLocation(_jsLocationForSocket);
+    _serverHostForSocket = [[RCTBundleURLProvider sharedSettings] packagerServerHost];
+    _socket = socketForLocation(_serverHostForSocket);
     _socket.delegate = self;
     [_socket start];
 
@@ -79,10 +79,10 @@ - (instancetype)init
   return self;
 }
 
-static RCTReconnectingWebSocket *socketForLocation(NSString *const jsLocation)
+static RCTReconnectingWebSocket *socketForLocation(NSString *const serverHost)
 {
   NSURLComponents *const components = [NSURLComponents new];
-  components.host = jsLocation ?: @"localhost";
+  components.host = serverHost ?: @"localhost";
   components.scheme = @"http";
   components.port = @(kRCTBundleURLProviderDefaultPort);
   components.path = @"/message";
@@ -118,15 +118,15 @@ - (void)bundleURLSettingsChanged
     return; // already stopped
   }
 
-  NSString *const jsLocation = [RCTBundleURLProvider sharedSettings].jsLocation;
-  if ([jsLocation isEqual:_jsLocationForSocket]) {
+  NSString *const serverHost = [[RCTBundleURLProvider sharedSettings] packagerServerHost];
+  if ([serverHost isEqual:_serverHostForSocket]) {
     return; // unchanged
   }
 
   _socket.delegate = nil;
   [_socket stop];
-  _jsLocationForSocket = jsLocation;
-  _socket = socketForLocation(jsLocation);
+  _serverHostForSocket = serverHost;
+  _socket = socketForLocation(serverHost);
   _socket.delegate = self;
   [_socket start];
 }