Add a preference for last online signin

The time of last signin was stored in local state
which is not fully available for ephemeral users which could
lead to undesirable behaviour. In this bug the policy values
would switch to default, because ephemeral user cannot read
from local state.

Bug: b:317146360
Change-Id: I044bbc59033a1c0a96dd67b9eaf08628a711c43a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6779260
Commit-Queue: Michael Slutskii <slutskii@google.com>
Reviewed-by: Andrey Davydov <andreydav@google.com>
Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Cr-Commit-Position: refs/heads/main@{#1492873}