Consolidate JNI registrations for mojo system.

mojo/android/system/mojo_jni_registrar.h

+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_ANDROID_SYSTEM_MOJO_JNI_REGISTRAR_H_
+#define MOJO_ANDROID_SYSTEM_MOJO_JNI_REGISTRAR_H_
+
+#include <jni.h>
+
+namespace mojo {
+namespace android {
+
+// Register all JNI bindings necessary for mojo system.
+bool RegisterSystemJni(JNIEnv* env);
+
+}  // namespace android
+}  // namespace mojo
+
+#endif  // MOJO_ANDROID_SYSTEM_MOJO_JNI_REGISTRAR_H_