Google password compromised?
You should immediately factory reset your phone, then go to Google and go through the Google Security Check-up and verify EVERYTHING (don't skip any checks), and I would also recommend enabling 2-Step Authentication.

After that, setup your phone again and make sure to enable a lockscreen password, and possibly device encryption as well. Do not enable "Unknown Sources" without somekind of quality security package installed.

I would also go through everything on the Sign In &amp; Security page and look for any anomalies.