If the icon is there Avast is active it doesn't need to be animated to check the program is working. If it is there it is, or should be, working. The animation is, therefore, an unnecessary distraction only telling you when Avast is actively engaged in doing something and tells you about it. If that makes you feel more secure OK but it is unnecessary.
It might not even be there at all, animated or static, yet still be working perfectly happily if somebody, for some reason, has reset the Avast icon to "Always Hide" in Taskbar & Start Menu Properties.
Hover cursor over tray empty part of tray > Right mouse click > Properties > Customise.