Make addCookies method public. (facebook#27512)

Summary:
react-native-cookie-store wants the ability to set custom cookies on Android. We use ForwardingCookieHandler to mange the cookies. Exposing the `addCookies` method will allow the module to provide the same functionality on Android.

safaiyeh/react-native-cookie-store#1

## Changelog

[Android] [Changed] - Expose addCookies method
Pull Request resolved: facebook#27512

Test Plan: N/A

Differential Revision: D19236309

Pulled By: cpojer

fbshipit-source-id: bf1a0730165456c34c5bf432ac370176a881cbcf

Author: safaiyeh

ReactAndroid/src/main/java/com/facebook/react/modules/network/ForwardingCookieHandler.java

@@ -124,7 +124,7 @@ public void destroy() {
     }
   }
 
-  private void addCookies(final String url, final List<String> cookies) {
+  public void addCookies(final String url, final List<String> cookies) {
     final CookieManager cookieManager = getCookieManager();
     if (cookieManager == null) return;