Consolidate OkHttpClient creation

Summary:
Consolidates creation of OkHttpClients used by RN panel apps into PanelAppOkHttpClientProvider. This diff adds no functional changes; however, a followup diff will leverage this to add an Interceptor for overriding the network tier.

Changelog:
[Internal] - Enable passing in custom OkHttpClient to NetworkingModule

Reviewed By: rshest

Differential Revision: D41621244

fbshipit-source-id: 8954f9adc6a0cfdf6312678e2dbd6be8ef9aa5ca

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

@@ -109,7 +109,7 @@ public final class NetworkingModule extends NativeNetworkingAndroidSpec {
   private final List<ResponseHandler> mResponseHandlers = new ArrayList<>();
   private boolean mShuttingDown;
 
-  /* package */ NetworkingModule(
+  public NetworkingModule(
       ReactApplicationContext reactContext,
       @Nullable String defaultUserAgent,
       OkHttpClient client,