Call ConfirmAuthToken when upload is done

ConfirmAuthToken will set secret used to generate provided token as
primary.

BUG=25766813

Change-Id: Ib00be7d1a64db078835c78e51d7a19c4780f3a06
Reviewed-on: https://weave-review.googlesource.com/1933
Reviewed-by: Vitaly Buka <vitalybuka@google.com>