Security is not an app. It's a habit. No app is a security 'solution'. But it's not hard to keep your phone secure. Android is a pretty secure system by design.
The biggest security risk to Android is Android users. Android users can also be the biggest security asset.
1. Use a password or pattern lock to keep people out.
2. Be very careful where you get software. Use only very trustworthy sources, basically the Play store and Amazon.
3. In Settings > Security > Device administration leave Unknown sources disabled. If you must enable it to do an install don't forget to disable it when done.
4. IMHO antivirus is kinda worthless. Instead of antivirus apps install Common Sense. Use it at all times. That is the most effective protection.
5. Do not root the phone. Rooting breaks basic Android security features such as user permissions and sandboxed apps.
6. Pay attention to permissions requested by apps during installation.
7. Use 2-step authentication.
Follow these simple steps and you will have no problems. You might consider encrypting the device but there are pros and cons to that.