Refactor card unmask flow metrics

Move all card unmask flow related metrics into card_unmask_flow_metrics.*

Update all refs accordingly.

Here's a diff of the moved code in .h file:
https://diff.googleplex.com/#key=gqRgb6PEDftr

Here's a diff of the moved code in .cc file:
https://diff.googleplex.com/#key=Nnw1KCPSzy6r

Bug: 1401506
Change-Id: I8e9d7ee72f663f562481f52aaba7e261c7dd33c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4112669
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Commit-Queue: Shikai Jin <shikaijin@google.com>
Reviewed-by: Siyu An <siyua@chromium.org>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1085752}