extract_utils: Fix pinning when not cleaning vendor dir

Skip the extraction of pinned files if the ones currently available
have the expected sha1. If we don't, we will overwrite pinned files
with potentially incorrect files when the current vendor files are
not moved to a temporary directory (i.e. when not cleaning vendor).

Change-Id: I640d6bf2ed98eb366a4df17f0ebeaec81cb5274b