Move kPrivateMetricsFeature to private_metrics

This feature will be used by generic Private Metrics code, not only DWA.
This patch moved kPrivateMetricsFeature to private_metrics under
:private_metrics_features.

Bug: 433731340
Change-Id: Ic0cbcfe12cf6c2a7e14d24ff00be10cc3d7f45c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7021143
Commit-Queue: Kamil Jarosz <kjarosz@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Jay Zhou <zhouzj@google.com>
Cr-Commit-Position: refs/heads/main@{#1533591}
NOKEYCHECK=True
GitOrigin-RevId: e7004c0dcd3153bc6ff3246207c69a9a591cd6d8