Android is clearly a goldmine. The goldmine is the incremental search revenue Google gets from controlling the Android platform. Why do you think the search volume on mobile is a hockey stick at Google? Literally every single android device has a dedicated search button on it; iOS devices do not. Which device do you think has higher searches per user? The incremental search volume (and search ad revenue) is enough to justify the entire Android investment.
More importantly, without Android, Apple had the ability to lock out Google from mobile search by changing the default search provider to non-Google. The lost revenue if Apple changes its mind and Google is locked out of mobile search? There's your 2nd goldmine.
The argument that Android is not a money maker is like the argument that Gillette selling razors below cost is bad idea. At the end of the day both Gillette and Google make a tons of money off selling something that isn't exactly the main product, but is peripheral to it. In Gillette's case it's razor blades, in Google's it's search.