The main thing with Android security is being careful where you download apps from. There are no true "viruses" for android, i.e. malware that can propagate itself. What you need to be wary of is trojans: malware hidden inside innocent-looking apps. If you only download from a curated source like the Play Store you are unlikely to encounter malware (occasionally a new one slips in, but it's rare and is removed on discovery). If you download apks from random sites on the internet then the risk is a lot higher. If you install apps from sites that offer paid apps for free then you are at a very high risk of infection: by definition those sites are already engaged in or tolerating one type of criminal activity, after all, so it should be no surprise if they are involved in (or just not looking for) others.
I do not have one of those apps on my phone, but I only install from the Play Store, the F-Droid repository, occasionally apkmirror if I need an old version of something.
If you feel you need something like that, I'd personally stick to an outfit who have an established reputation in computer security (someone like MalwareBytes, Avast, etc) and who have a business model that you can understand (e.g. funded by selling premium versions). A company who produce a lot of apps, which require a lot of updates (which a security app does) and yet has no visible source of income should always ring alarms. In the case of Cheetah Mobile they have openly said that they make their money from their customers' data, and given that a security app requires access to everything in order to work I most certainly would not install something like that from a company of that sort.
Note that if malware is new then it's likely that your security app won't know how to detect it, so having one of those things installed isn't a guarantee of safety (though it obviously reduces the risk of infection). Thinking about what you install and where from is the most effective protection, and a security app should be seen as a supplement to that rather than a replacement.
