[Uno-D] Store the first Uno bubble decline time

Store the time in the account pref through the SigninPref structure.
This time will be used to reassess when the re-promt the users with
the Uno bubble in a followup CL.
This pref is cleared when the user explicitly chooses not to sign in
to Chrome from a web sign in event through the Setting page.

Bug: b:345192629
Change-Id: Ie8a15a52a49ed6e94da1b04c373bda5c05c81710
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607671


Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1312818}