After playing with Android for quite some time, I have really realized how AV apps on Android are almost really useless. If you read permissions and understand them, and you don't side load apps then it is better not to pay for one or even get a free version. Except if you want to use other features present in that app like Anti theft, firewall and few others. All this can be offered for free by Avast. Also in the newly released Avast for android there is a web based anti theft along with SMS commands.
From my experience, the only malware type that I got installed on my Android tablet is the Adware called Airpush, which displays ads in the notification bar. There is no way to figure that out from the permissions, you need to take your time to read some reviews in the play store.
Android AV programs don't have root (admin) access, they only search the apps for infections, if the app installed was not detected, then it can't protect from changes to the system.