New "online-updates-timestamp" gsetting for online updates tracking
"online-updates-timestamp" will denote the last time the system was online and got any update. It will play as one of the factor to drive pending updates notification logic in gs-update-monitor (See no_updates_for_a_week). This timestamp is set by plugin-loader whenever there is a update or upgrade operation (that succeeds).
The reason we introduce a new gsetting instead of using a similar one "install-timestamp is that, "install-timestamp" is used for scheduling offline PackageKit update notifications. If we reuse this one for flatpak updates as well, this can lead to PackageKit updates never getting any notifications (as flatpak updates are autoinstalled).
Based on Endless dev ticket: https://phabricator.endlessm.com/T22720