add the ability to disable notification when suspending because of inactivity

Some phones that use the Gnome-based Phosh stack. Due to short battery life on most phosh-based phones, many of these use suspend to lengthen battery life. People using these phones often set the suspend timeout to 15 minutes (or even less). However, every time the phone is about to suspend, we get a notifiction telling us that the phone is about to suspend. Given that suspending is a normal activity for phones, one that the user should barely even notice, I think we should provide a way to disable the constant notifications from G-S-D telling the user that the phone is about to suspend.