Yes, Android devices have serious security problems. There’s Android malware out there — mostly outside the Google Play Store. The biggest problem is that most Android devices don’t get security updates. Android antivirus apps aren’t a solution to these problems.
Security companies have been pushing their Android antivirus apps, using the concern over the Stagefright exploit to sell security software. But Android antivirus apps aren’t going to help you here.
http://www.howtogeek.com/232436/android-has-a-big-security-problem-but-antivirus-apps-cant-do-much/
A very interesting read, especially the section titled, "So What Do Android Antivirus Apps Do?".
Also:
It’s understandable that many Android users, coming from Windows, would think to install an antivirus application. After all, many of these applications are made by companies that also create Windows antiviruses. But those antivirus apps don’t function like Windows antivirus software and don’t have enough permissions to really secure your device. Android already has more comprehensive antivirus-style protections baked into the operating system.