Add MockEndpointFetcher and AccountCheckerTest

This CL contains the following:
* Add MockEndpointFetcher for easy test so we don't have to create a
mock class in each test file. Related tests are simply polished.
* Add test for AccountChecker.
* Within AccountChecker, if the email pref change is triggered by
aligning with the server fetched value, we shouldn't send the updated
value to the server again.

Low-Coverage-Reason: mock_endpoint_fetcher.cc is for test only.

Bug: 1351599, 1366165
Change-Id: I94ecdd1162a1fe07a9a89868830d35e65b1f3ea8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4035239
Commit-Queue: Zhiyuan Cai <zhiyuancai@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Ayman Almadhoun <ayman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1095792}