If i can recommend you some antivirus for android, try F-secure, ESET or Bitdefender.
I noticed something in the way some of these antiviruses work.Those that use the cloud such as Bitdefender, there is a sort of latency in detection.When u download a file,it will not be detected.Once you try to install the apk it detects the malicious file.
Products like Eset and F-Secure on the other side,use downloaded definitions and cloud.Once you download a malicious file, a bit like on Pc, you get an alert instantly.But they will use more memory,space and resources on your phone.F-Secure is very well designed, from the GUI to the notifications,has a secure browser and a mode specially for internet banking.But it can take up to 90mbs on your device.
Emsisoft is same as Bitdefender, but cost twice less.If you have time,try them.