Reland "metrics: introduce ExternalDisplayEventsObserver"

This is a reland of commit 0211765461b3132d491cfc0adb502ff0bcc0e912

1) Switch to use DisplaysTelemetry instead of DisplayInfo
2) Add chromeos::features::kExternalDisplayEventTelemetry
3) Allow ExternalDisplayEventsObserver only if the feature
   enabled.
4) Update tests

Original change's description:
> metrics: introduce ExternalDisplayEventsObserver
>
> Introduces external display events observer and new metrics events that
> passes connected/disconnected external display via Telemetry API.
>
> Bug: 327532834
> Change-Id: Ie92106031ffb4cad1526e9a999fa45c1cfcea082
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6489171
> Auto-Submit: Maksim Sisov <msisov@igalia.com>
> Reviewed-by: Leonid Baraz <lbaraz@chromium.org>
> Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1453327}

Bug: 327532834
Change-Id: I3fdc0ee164846a18ae748b885836f86dfeb3d2d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6543589
Reviewed-by: Leonid Baraz <lbaraz@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1468596}