Add an empty Android.mk

This is to prevent Android.mk in subdirs from being parsed in the
master-plus-llvm branch.

Cherry-picked from 47a7a80b45aab2306ca73203a2df6ea76771393c in r16
NDK.

Test: 'python toolchain/llvm/android/test_compiler.py --build-only ./' in master-plus-llvm branch

Android.mk

diff --git a/Android.mk b/Android.mk
new file mode 100644
index 0000000..a551cd7
--- /dev/null
+++ b/Android.mk
@@ -0,0 +1,17 @@
+#
+# Copyright (C) 2017 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.
+#
+
+# Intentionally empty to prevent subdir Android.mk loading