Add script to mirror AOSP projects.

repo --mirror doesn't really properly mirror - it fetches projects included
in manifest for specific Android version, so checkout out other Android
version from such mirror will require net access still. This scripts instead
mirrors all projects repos available in AOSP (filtered, or just
specific projects if needed).
And docstring for script, and mirror into "android-mirror" subdir.

Obviously, android-mirror can be a symlink to existing mirror too.
Add script to fetch particular AOSP rev from local mirror.
Add script to build particular AOSP rev (fetches as needed).