Usually, I find full screen ads on my Android mobile. Also, apps like task bucks automatically install on my smartphone.
This can be caused by either an app you installed or one that comes with your phones ROM, especially if your phone is a really cheap one, such as knock offs from China.
- Preinstalled app causing the issue
I experienced this when I bought a phone to fiddle with, a cheap one. Check if you have an app entitled: com.android.systemservice running the service "systemService" This is a trojan and uses Webkit to display fullscreen ads and installs apps without your permission. This is rather common thus can be the culprit.McAfee reported on this here. To uninstall it, you will have to root your phone and use an app such as `System app remover.
- Apps you installed
Happens often, especially if you download APKs off unknown sources other than the Play Store. Either uninstall an app you recently installed and check if the adds appear, as user4847410 suggested. Alternatively, use 360 Security, Malwarebytes or anti-adware (play.google.com/store/apps/details?id=com.keerby.adaware&hl=en)
Proxyserver and proxyhandler can't be the cause as they are core parts of Android.