Add NOTICE files for our projects.

Bug: http://b/25592257
Change-Id: I4bdd59c30ce1ecca68c7aa133b4a32afb72927cf

ndk/sources/android/ndk_helper/NOTICE

@@ -0,0 +1,13 @@
+Copyright (C) 2016 The Android Open Source Project
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.