buffet: Refetch access token when expired. BUG=chromium:420580 TEST=cros_workon_make buffet --test&&manual Change-Id: I17c5eb2ef86991baed1a700007a11537bb07104d Reviewed-on: https://chromium-review.googlesource.com/224940 Tested-by: Anton Muhin <antonm@chromium.org> Reviewed-by: Alex Vakulenko <avakulenko@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org>