Create an experimental Android NDK.

Following files were split:
sdk/android/native_api/jni_helpers.h
  -> sdk/android/native_api/jni/java_types.h
sdk/android/native_api/jni_helpers.cc
  -> sdk/android/native_api/jni/java_types.cc

Skipping presubmit to avoid changing moved code.

Bug: webrtc:8769

Change-Id: I0ef0f6b297b5002322915660d26cca33e91ff05b
No-Presubmit: true
Reviewed-on: https://webrtc-review.googlesource.com/40800
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21799}
diff --git a/sdk/android/native_api/jni/jvm.h b/sdk/android/native_api/jni/jvm.h
new file mode 100644
index 0000000..00bce67
--- /dev/null
+++ b/sdk/android/native_api/jni/jvm.h
@@ -0,0 +1,21 @@
+/*
+ *  Copyright 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef SDK_ANDROID_NATIVE_API_JNI_JVM_H_
+#define SDK_ANDROID_NATIVE_API_JNI_JVM_H_
+
+#include <jni.h>
+
+namespace webrtc {
+// Returns a JNI environment usable on this thread.
+JNIEnv* AttachCurrentThreadIfNeeded();
+}  // namespace webrtc
+
+#endif  // SDK_ANDROID_NATIVE_API_JNI_JVM_H_