Makes file download progress be indicated exclusively

fixes CNVS-16870

Test Plan:
  - Go to New Files, using Screenreader
  - Select several files (a lot for fun)
  - Download as zip
  - The file percentage should be read exclusively rather than
    constantly adding to the message being read

Change-Id: Icb2a459ccf29ed930a7e184cd4e7b05e8ba694fc
Reviewed-on: https://gerrit.instructure.com/44375
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>