Add back ability to customise OkHttp client

Prior to 0a71f48, users could customise
the OkHttp client used by React Native on Android by calling
replaceOkHttpClient in OkHttpClientProvider.

This functionality has a variety of legitimate applications from
changing connection timeouts or pool size to Stetho integration. The
challenge is to add back support for replacing the client without
causing a breaking change or undoing the problems @olegbl sought to
address in his original commit. Introducing a client factory achives
these aims, it adds a new, backwards compatible interface and is called
each time a client is requested rather than re-using the same instance
(unless you explicitly want this behaviour, in which case you could
replicate it using a static class property inside your custom factory).

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

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.modules.network;
+
+import okhttp3.OkHttpClient;
+
+public interface OkHttpClientFactory {
+    OkHttpClient createNewNetworkModuleClient();
+};

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

@@ -32,6 +32,13 @@ public class OkHttpClientProvider {
   // Centralized OkHttpClient for all networking requests.
   private static @Nullable OkHttpClient sClient;
 
+  // User-provided OkHttpClient factory
+  private static @Nullable OkHttpClientFactory sFactory;
+
+  public static void setOkHttpClientFactory(OkHttpClientFactory factory) {
+    sFactory = factory;
+  }
+
   public static OkHttpClient getOkHttpClient() {
     if (sClient == null) {
       sClient = createClient();
@@ -46,6 +53,10 @@ public static void replaceOkHttpClient(OkHttpClient client) {
   }
 
   public static OkHttpClient createClient() {
+    if (sFactory != null) {
+      return sFactory.createNewNetworkModuleClient();
+    }
+
     // No timeouts by default
     OkHttpClient.Builder client = new OkHttpClient.Builder()
       .connectTimeout(0, TimeUnit.MILLISECONDS)