Short answer in the free version of avast you can't - there is an option to stop these promotional popups.
The popup is there even if you already have the Avast Mobile Security app installed on your android phone - there is no way for avast to know if you even have an android phone - so it isn't specifically targeted.
I have avast free and this didn't used to be that frequent, but of late it is getting more frequent. However, it doesn't bother me that much on my 1920X1200 screen.
However, I'm just browsing, etc. and it doesn't get in the way - if you are gaming I can see it being a pain. If you are full screen games should block this I believe. Or try setting Silent/Gaming mode in the avastUI > Settings > General section and see if that helps.