Revert "Do not emit initial (false) value of content filter observers."

This reverts commit 0ad14b3a6e42d57a57f4d0939684db177232eb44.

Reason for revert: not a proper fix 

Original change's description:
> Do not emit initial (false) value of content filter observers.
>
> This is causing noise in metrics (that's a symptom), because the
> supervised user service bootstraps content filter observers that
> broadcast "disabled" settings that are already disabled (their initial
> value). This it not a no-op for supervised users: it removes their
> settings set from AccountTrackerService which loads account information
> from disk cache. This is later fixed when account information is loaded
> again from the remote (but yields extra metric records, because eg. the
> URL Filter is enabled, disabled and then again enabled).
>
> Assertions added to supervised_user_service will prevent starting the
> browser with supervised account and device filters at the same time -
> these are mutually exclusive.
>
> Bug: 431949472
>
> Low-Coverage-Reason: OTHER Uncovered jump-out
>
> Change-Id: I5c100f6065621931fc0c6b45b1da6dd0639bfa73
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6764626
> Reviewed-by: Nicolas Dossou-Gbété <dgn@chromium.org>
> Commit-Queue: Tomek Jurkiewicz <tju@google.com>
> Cr-Commit-Position: refs/heads/main@{#1488444}

Bug: 431949472
Change-Id: Id5a6d63e812ef66fbc2205230914ea2626b26cfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6771768
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Tomek Jurkiewicz <tju@google.com>
Cr-Commit-Position: refs/heads/main@{#1489423}