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.
1 job for fix-install-timestamp in 4 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
passed #196410