Google doesn't have a strategy to serve ads on android? sure they do, it's called a web browser. Google has the largest ad network on the internet, and just about every time you view a webpage google gets a cut of it. It doesn't matter whether you use an android device or an iOS device or a windows computer or a potato, google makes money.

if google can encourage people to use the internet more by selling smartphones, they win. The point of android is not to cannibalize iOS market share, it's to eliminate the dumbphone. They don't want anybody to ever be without internet.