Add hybrid class to hold native delta client

Summary:
Adds hybrid JNI wrapper class that can hold a native delta client.

This class allows Java code in the `devsupport` package to hold onto a native delta client across reloads.
It also takes care of passing `okhttp` response objects to native code, where we build a `std::string` to hold JSON messages coming from Metro, and parse them with `folly::parseJson`.

In a follow-up, we will switch to a streaming-friendly binary format that will make allocating memory for the whole message unnecessary.

Reviewed By: fromcelticpark

Differential Revision: D7845138

fbshipit-source-id: 7a29b30645331addf843097dd0d05c03b3143991

ReactAndroid/src/main/java/com/facebook/react/bridge/NativeDeltaClient.java

@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) 2018-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.bridge;
+
+import java.nio.channels.ReadableByteChannel;
+
+import com.facebook.jni.HybridData;
+
+public class NativeDeltaClient {
+  static {
+    ReactBridge.staticInit();
+  }
+
+  // C++ parts
+  private final HybridData mHybridData = initHybrid();
+  private native static HybridData initHybrid();
+
+  public native void reset();
+  public native void processDelta(ReadableByteChannel deltaMessage);
+}