Since Android Jelly Bean, Google has implemented a system that allows notifications to have various levels of priority based on their current relevance. This allows messaging/chat to have a higher priority so they will be seen, while other, less important notifications will be further down in the notification drawer. In addition notifications with the priority of "MIN" will be hidden from the notification bar completely, but will still be present in the notification drawer. This method should allow Avast! to maintain a persistent notification (too keep Android from killing it) while keeping the unimportant icon from cluttering my notification bar.
If you want to see this in action, the Tasker app is a great example. By selecting the blank icon for the notification, Tasker actually sets the priority to MIN, thus hiding the notification from the bar while still preventing Android from killing Tasker.
Here's a screenshot of what that looks like in practice:
If you want more information about Android notifications see the design guidelines (
http://developer.android.com/design/patterns/notifications.html) and the programming (
http://developer.android.com/guide/topics/ui/notifiers/notifications.html) straight from Google.
