fix double-_localstorage_ in Attachment#infer_namespace

test plan:
 - in a Rails development environment using local file storage,
   1. use the content migration API to import the contents
      of a zip file into a folder in a course
      (use zip_file_importer as the migration_type, and set
       settings[folder_id] to the id of a folder in the course)
   2. repeat step (1), but specify a different folder id
   3. confirm the files can be downloaded and/or deleted in the
      second folder without a "Cannot find Account with id=0"
      error

fixes CNVS-15726

Change-Id: Icb73efcf5b933f4a6b9dea9c0e2c884bfb82afd5
Reviewed-on: https://gerrit.instructure.com/41583
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>