[QuickStart]: Add UMA metric for finch analysis

This metric will help us distinguish users with m126+ pre-installed
to prevent data contamination during Quick Start experiment analysis.

Design: go/cros-quick-start-experimentation-plan (See "Preventing
contaminated data")

Test: manual test with atlas and pixel8
Change-Id: I312270f118477214afbe05773437d5090aa8ce89
Bug: b/333941437
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5460434


Reviewed-by: Renato Silva <rrsilva@google.com>
Commit-Queue: Brittany Hartmire <bhartmire@google.com>
Cr-Commit-Position: refs/heads/main@{#1289438}