I use Avast Free on all my computers but I don't bother on my phone. Hardly worth it and as with all the AV companies, the dumb extra "Free" programs they add on if you click them just suck resources and give you no real advantage.
Avast, AVG, McAfee free versions all operate in the same way - throw you offers and before you know it you have joined a trial 30/60 day and then they take payment from your existing payment method. Best advice I can give you is log into their online account and cancel the auto Renewals!
That said, many of my customers subscribe to the Avast Paid for versions at about £10-£20 a year and are comforted by that fact. Often the mobile versions come free! Yet still a couple end up with paid for subscriptions to the "VPN PRO", The "Spam PRO" thinking they were renewing their "AntiVirus PRO" - be careful out there! LOL