I think Facebook is likely the culprit. I bought a brand new phone, the Facebook app is of course among the bloatware, but it hadn't been too much of a problem so I let it be. Ended up logging in because that's one of my methods of contact, for whatever reason they apparently can't include a messenger within the app so I had to install messenger. Used them for a while, suddenly I start getting that popup even though I denied it twenty times at least, and my phone's got major lag. I changed a took of settings, notification permissions, it was through Google Play Services but changing Google Play Services notification settings didn't fix it. The Facebook app does this funny thing where when I hit the button that shows the apps I opened and didn't close, Facebook didn't show up. Like I never opened it, sometimes Chrome would appear in its' place in that list. That makes me think it operates through other apps. If it does that why wouldn't it be sending popups through another app? In settings when you go to disable the Facebook app it tells you that some things might not work properly might misbehave. Wouldn't that mean it's apart of how the phone operates itself or had something to do with it? That combined with not having the option to uninstall it, seems like it to me. I uninstalled Messenger, disabled and force stopped Facebook, uninstalled Facebook's updates, force stopped and disabled it again and my phone is working like it did before I used them. Haven't had to deal with that annoying popup since and I didn't even have to restart my phone. I'm not sure if Messenger was causing the problem, but I'd rather not have it take up space and lag my phone, especially when I can use it online in a browser. Good luck guys! Hope this helps!