[Amount Extraction][Flag] Add flag for the eng and QA to test the search
quality of the amount extraction engine

This flag will never be enabled on production, it is only manually
enabled by Eng and QA during the canary testing stage for the purposes
of improving the amount extraction search quality.

Bug: 398851089
Change-Id: Icd62d1e235e60e5973b7e0b6b4a59ab222fd70f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6341670
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Siyu An <siyua@chromium.org>
Commit-Queue: James Sun <jamessun@google.com>
Cr-Commit-Position: refs/heads/main@{#1433006}