Set install-timestamp gsetting when we update or upgrade

install-timestamp is the gsetting which drives app-center
notifications for pending updates. However, the logic for
driving notifications was already present in gs-update-monitor
(see no_updates_for_a_week) but it turns out that install-timestamp
gsettings was not being set at appropriate places when the user
updates or upgrades. We fill this gap by relying on plugin-loader's
logic because that drives update or upgrades at a macro level.

https://phabricator.endlessm.com/T22720
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build-gnome-software

00:04:37