Add an empty Android.mk to shadow buildcmds/Android.mk

buildcmds/Android.mk is only meant to be used by mm from run-tests.py,
add an empty Android.mk at the top level to hide it.

Change-Id: Ib608e908f5fd0630f91b355635501bde8803a000
diff --git a/Android.mk b/Android.mk
new file mode 100644
index 0000000..f10c507
--- /dev/null
+++ b/Android.mk
@@ -0,0 +1 @@
+# This empty Android.mk file is required to shadow buildcmds/Android.mk