MDL-36943 Do not notify about already installed updates

MDL-36943 Do not notify about already installed updates

Before this patch, the available_update_checker::cron_notifications()
method accessed the availableupdates property. But for plugins, that property
basically contains the most recent version available. So we were missing
the check against the actual version installed.

The fix was simple - obtain available updates via the available_updates()
method that performs the check.