Move reachability flags initialization to +initialize from +load (#5477)

Author: mikehaney24

GoogleDataTransport/GDTCORLibrary/GDTCORReachability.m

@@ -42,7 +42,7 @@ @implementation GDTCORReachability {
   GDTCORNetworkReachabilityFlags _callbackFlags;
 }
 
-+ (void)load {
++ (void)initialize {
   [self sharedInstance];
 }