Prevent emitting UMA due to our own experiment setup process.

When setting up the PreinstalledWebAppWindowExperiment for "Window"
user_group, we call `SetSupportedLinksPreference` on each preinstalled
web app, then start observing for user changes to the supported links
preference via `OnPreferredAppChanged`. In some cases (especially
Lacros) we start listening before those initial values are set, so we
receive observations from our own setup (only for apps that actually
changed preference) and record these as metrics (as if user actions).

This CL filters out those observations from our own setup by ignoring
the first observation for each set-up app, if it shows the app being set to the experiment value (ie. is_preferred_app==true because we only
SetSupportedLinksPreferenceForPreinstalledApps when users are in the
"Window" user_group).

Bug: 1427622
Change-Id: I448286a1694ea5829b94f9f296f768f1389b90b5
Fixed: 1427622
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4374223
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1125218}