- Split the download part into a separate script to so docker keeps that
step in cache and avoid redownloading it.

- With API level >= 28 libiconv is not needed anymore because it's part
of Android's libc.

- Generate standalone toolchains to reduce the docker image size. It's
also easier because it doesn't need to pass sysroot args.

- Use clang compiler because gcc is deprecated in this Android NDK and
will be removed in the next release.