use the file uid/gid/mode saved in the target_files

When building a bootable image with mkbootfs, use the set of file
metadata that's stored in the target_files zip (when available),
rather than whatever is built into the mkbootfs binary at the time the
image is built.

Bug: 6435132
Change-Id: If6c59149bdbcc9a67e5ab9161398f355bd1f511d