[Android] add toplevel Android.mk to stop breakage

[nickdesaulniers]

Summary

Android is currently moving over to building AOSP's LLVM release from
the monorepo. In testing, we were hitting breakage due to the Android.mk
file under pstl/ (parallel STL). Previously, Android was not including
pstl/ as a separate repo and now with the monorepo we get it no matter
what (and will probably use it soon).

The issue for Android's build system is that it recursively looks for
Android.mk files, so it picks up the one under pstl/ even though we
don't want to build it just yet.

The sugguested fix from Android's build team is to add an empty
Android.mk to the top level directory, which will prevent the build
system from recursing further, finding the one in pstl/, and erroring
out.

Link: https://android-review.googlesource.com/c/platform/manifest/+/977993

[nickdesaulniers]

Looks like this is not the correct solution. See also: https://android-review.googlesource.com/c/toolchain/llvm-project/+/979791

Android.mk

+#===-- Android.mk --------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+#
+# Intentionally empty to prevent Android's build system from loading deeper
+# Android.mk files such as from pstl.