[Signals Reporting] Move active trigger into the active config

ReportScheduler currently keeps two instance variables, an active trigger and an active report generation configuration.

Since theses two variables need to be used in pair and kept in sync, it makes sense to just make the active trigger a part of the generation configuration.

Change-Id: Ic626220466a5baecd40cce658cbac1bea8076a7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532205
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Commit-Queue: Zonghan Xu <xzonghan@chromium.org>
Reviewed-by: Sébastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459643}