attestation: Use VPD cache.

BUG=chromium:783127
TEST=PreCQ passes.

Change-Id: I75a200f42bb7710e478e96c31fc02b07cc570fd8
Reviewed-on: https://chromium-review.googlesource.com/766587
Commit-Ready: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Yves Arrouye <drcrash@chromium.org>