It doesn't have to be an apk app that causes the adds.
It will likely be an app that you downloaded just before or about the time that the adds started that is causing the problem.
Was there anything you downloaded(doesn't have to be an APK) in the past few days? Maybe go back to check your downloads folder. @Shaggyskunk1 and @Ray2 are correct, Samsung doesn't have native apps inside their UI or system. Please keep us posted!