Support cookies on Android

Summary: This adds a persistent cookie store that shares cookies with WebView.

Add a `ForwardingCookieHandler` to OkHttp that uses the underlying Android webkit `CookieManager`.
Use a `LazyCookieHandler` to defer initialization of `CookieManager` as this will in turn trigger initialization of the Chromium stack in KitKat+ which takes some time. This was we will incur this cost on a background network thread instead of during startup.
Also add a `clearCookies()` method to the network module.

Add a cookies example to the XHR example. This example should also work for iOS (except for the clear cookies part). They are for now just scoped to Android.

Closes #2792.

public

Reviewed By: andreicoman11

Differential Revision: D2615550

fb-gh-sync-id: ff726a35f0fc3c7124d2f755448fe24c9d1caf21

Libraries/Network/RCTNetworking.android.js

@@ -40,6 +40,10 @@ class RCTNetworking {
   static abortRequest(requestId) {
     RCTNetworkingNative.abortRequest(requestId);
   }
+
+  static clearCookies(callback) {
+    RCTNetworkingNative.clearCookies(callback);
+  }
 }
 
 module.exports = RCTNetworking;