[Floating SSO] Fix a crash when accessing store before initialization

When cookies were added or deleted, FloatingSsoService would access
local Sync store to check if corresponding change needs to be propagated
to the server. This could happen early on startup before the store is
initialized, which resulted in a crash.

This cl moves this logic from the service to the bridge (i.e. from
FloatingSsoService::OnCookieChange to
FloatingSsoSyncBridge::AddOrUpdateCookie and
FloatingSsoSyncBridge::DeleteCookie) where we already handle
uninitialized store. Coincidentally, this also uncovered the fact that
we were missing the same optimization on the service startup when
processing all locally existing cookies. Now we also don't send
unnecessary changes there (hence the change in the browser test
FloatingSsoStopsListeningAndResumes).

Testing this optimization became much easier: instead of a browser test
under FloatingSsoWithMockedBridgeTest we now have just a few extra lines
in bridge unit tests AddOrUpdateCookie and DeleteCookie.

Bug: b:405226650
Change-Id: I9c480c1bb1e0fa8fa4c3137bae39e921e84f2b5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6469795
Reviewed-by: Maria Petrisor <mpetrisor@chromium.org>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Andrey Davydov <andreydav@google.com>
Cr-Commit-Position: refs/heads/main@{#1449983}