Do not block FirebaseAuth thread execution while validating phone number.

Author: yramanchuk

FirebasePhoneAuthUI/FUIPhoneEntryViewController.m

@@ -196,7 +196,7 @@ - (void)onNext:(NSString *)phoneNumber {
     if ([NSThread isMainThread]) {
       completionBlock();
     } else {
-      dispatch_sync(dispatch_get_main_queue(), completionBlock);
+      dispatch_async(dispatch_get_main_queue(), completionBlock);
     }
   }];
 }

FirebasePhoneAuthUI/FUIPhoneVerificationViewController.m

@@ -163,7 +163,7 @@ - (IBAction)onResendCode:(id)sender {
     if ([NSThread isMainThread]) {
       completionBlock();
     } else {
-      dispatch_sync(dispatch_get_main_queue(), completionBlock);
+      dispatch_async(dispatch_get_main_queue(), completionBlock);
     }
   }];
 }