[People Service] Fix `hasActiveNotifications` to track when notifications are modified

Instead of tracking the number of active notifications, we track notifications for a package by their id. This is more robust and allows People Service to keep track of notifications that are updated, such as Messages which updates an existing notification for new incoming messages.

This fixes an issue in which recent conversations are retained even if they do not contain active notifications.

Bug: 263634015
Change-Id: I187161391a44f9e03840b89d88c4d3c083e1b9bc