Google's customers include advertisers, and they include people paying them for hosting, and people paying them to buy apps, and people paying them to publish apps, people paying them for online services, people buying their hardware, etc.
Their revenue last year was 136 billions. Even if we charitable assume app developers spend 10 billions per annum in aggregate, that's only 10%. Ancillary revenue.
https://www.bloomberg.com/news/articles/2016-01-21/google-s-...
I read it as saying $31 billion in revenue since Android was released, not in 2016 alone. That was more than 10 years ago.
And of that $31 billion, $22 billion is profit.
Finally, a lot of that revenue came from payments made to Google by handset operators, not app developer fees.