Refactor code and create utils/aosp_build_tag.sh

[jcfr]

The idea is to avoid code duplication in tests/build-kitkat.sh and tests/build-lollipop.sh

Ultimately, it should allow to build any tag/branch listed on https://source.android.com/source/build-numbers.html#source-code-tags-and-builds

[kylemanna]

True. I assume that people really using this will fork the build scripts to something more suitable for their own builds. I keep them around as a demonstration for those investigating the Docker image.

That said, de-duping the code is always nice.