Build overlaytests as part of the normal build

Test: make checkbuild
Change-Id: I1ebdbfd309246b24f1f64e3555c6dfdc592b1868

core/tests/overlaytests/Android.mk

@@ -1,4 +1,15 @@
-# Dummy makefile to halt recursive directory traversal.
+# 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.
 
-LOCAL_PATH := $(call my-dir)
-include $(CLEAR_VARS)
+include $(call all-subdir-makefiles)