In 2015, Google's revenue amounted to 74.54 billion US dollars. It was largely made up (90%) by advertising revenue, which amounted to 67.39 billion US dollars in 2015. [1]
A rough estimation is that 80% of Google's advertisement revenue comes from search. [2]
1: http://www.statista.com/statistics/266206/googles-annual-glo...
2: http://www.statista.com/statistics/343191/googles-digital-ad...
Many (most?) people primarily search from their phones now, so I imagine Android generates a lot of ad revenue. Android is even more important because most people don't know how to block ads on it, whereas desktop ad blocking has become mainstream.
http://www.businessinsider.com/oracle-google-generated-42-bi...
But while we can't take Oracle's statements at face value, how do you know Android isn't making Google money? It's also likely that without Android, Google's revenues would've declined a lot faster.
Even if say people would've still used Adsense/Admob on iOS or other mobile operating systems, look what Apple did - it allowed adblockers on its platform. Google doesn't allow them and Chrome, which is the dominant browser on Android doesn't support extensions, so no adblock there either.
its just that the mobile chrome doesnt support extensions. google isnt stopping anyone from a local "adblock" proxy or editing the phones host file.