PCScan: Refactor StatsCollector into a separate file

This is only a refactoring change, no changes in logic.

Bug: 11297512
Change-Id: Id048853cef3532689e8663fe016d690c55457b70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880666


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880996}