s/VLOG/LOG(WARNING)/ for certificateProvider logs

Switch the temporary verbose logging in the certificateProvider API
implementation to use LOG(WARNING) instead of VLOG, since the latter
was recently disabled for most of modules due to performance penalty
(see https://b/77729987).

These logs will be removed once we finish stabilizing the smart card
user login feature and collecting feedback from the early customers.

Bug: 1046860
Change-Id: I96223be406c267043d1995ccc8d905718dc465ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327930
Auto-Submit: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Fabian Sommer <fabiansommer@chromium.org>
Reviewed-by: Fabian Sommer <fabiansommer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793101}