My complain was about this policy let this happen. Obviously in a company like Google there should be company wise rule which won't let a stranger able to send notification to my phone.
Strangers have always been able to cause notifications to appear on your phone - notifications for SMS, social media, calls, email etc can all originate from people you don't know.
With my permission though. Those are expected and under my control. But in this situation it is not even expected to stranger to be able to share files with me and notify me. I dont even receive e mail for that instead direct notification to my phone.
Spam is not problem for me. What bothers me is just this stupidity comes from Google which unfortunately i rely on too much by using android.