I've added explanatory comments to mzExtractRecursive because
that function will live on as a utility even after we move the
zip format related logic to libziparchive.
bug: 19472796
(cherry-picked from commit c9ccdfd7a42de08c47ab771b94dc5b9d1f957b95)
Change-Id: I8b7fb6fa3eafb2e7ac080ef7a7eceb691b252d8a