Add jni-hack to RN OSS

Summary: See committed README.md. This is part of the migration to the jni implementation of CSSLayout.

Reviewed By: emilsjolander

Differential Revision: D4177009

fbshipit-source-id: f1860f5d4ffafa1375a9658227e0ac10b7df4845

ReactAndroid/src/main/jni/first-party/jni-hack/BUCK

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+# Copyright (c) 2014-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.
+
+cxx_library(
+  name = 'jni-hack',
+  force_static = True,
+  header_namespace = '',
+  exported_headers = [
+    'jni.h',
+    'real/jni.h',
+  ],
+  visibility = ['PUBLIC'],
+)

ReactAndroid/src/main/jni/first-party/jni-hack/README.md

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+This buck module exists so that targets that need to be built against both 1) Android (where we can and should use the Android NDK jni headers) and 2) the host platform(generally for local unit tests) can depend on a single target and get the right jni header for whatever platform they're building against automatically.

ReactAndroid/src/main/jni/first-party/jni-hack/jni.h

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+/**
+ * Copyright (c) 2014-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.
+ */
+
+#pragma once
+
+#ifdef __ANDROID__
+#include_next <jni.h>
+#else
+#include "real/jni.h"
+#endif