[libc++][Android] Add libcxx-builder-android Docker image (#69273)

Add a Dockerfile for a new Docker image, libcxx-builder-android, that
extends libcxx-builder with support for testing Android.

The image includes these things:

 * An Android Clang compiler and sysroot.

* The Android platform-tools (e.g. adb), so that an Android buildbot can
run programs on an Android device. At container startup, copy these
platform tools to an "android-platform-tools" Docker volume to share
them with an emulator container. This copying ensures that the emulator
and libcxx-builder containers avoid mismatched adb versions.

* Docker, so that an Android buildbot can manage a sibling Docker
container that runs the Android emulator.

Add an Android-specific run-buildbot-container script for local
development. Currently using this script requires building
libcxx-build-android and an emulator image locally.

Fixes: https://github.com/llvm/llvm-project/issues/69270
Differential Revision: https://reviews.llvm.org/D155271