PCScan: Hide singleton behind static interface

Simplify PCScan interface a bit by making all public functions static
and making Instance() private.

Bug: 11297512
Change-Id: Iaea3551d6d5209a095530f0adb714e5829818d68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2854636


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Owners-Override: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#876965}