Reduce visibility of HeaderUtil

Summary:
In an attempt to reduce footprint of React Native Android public APIs we are reducing visibility of classes and interfaces that are not meant to be used publicly OR are public but have no usages.
As part of our analysis, which involved looking for usages inside the Meta codebase and code search in OSS, we've detected that this class/interface is public but it's not used from other packages.

If you are using this class or interface please comment in this PR and we will restate the public access.

bypass-github-export-checks

changelog: [Android][Changed] Reducing visibility of HeaderUtil

Reviewed By: arushikesarwani94

Differential Revision: D49803294

fbshipit-source-id: 01ec431cf2addc8c0df0d729f5cc15c0334b4762

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/modules/network/HeaderUtil.java

@@ -13,7 +13,7 @@ package com.facebook.react.modules.network;
  * information. It is better to get 401 from the server in this case, rather than non descriptive
  * error as 401 could be handled to invalidate the wrong token in the client code.
  */
-public class HeaderUtil {
+class HeaderUtil {
 
   public static String stripHeaderName(String name) {
     StringBuilder builder = new StringBuilder(name.length());