That's not the way I see it. Google realizes that Apple with a strong majority share of the mobile market would be free to ignore Google entirely. They could severely reduce Google's ability to advertise by controlling mobile app ads. Google needs something like Android to ensure some fair play I believe. If mobile is to eclipse desktop usage, they can't be left out. But in any case Apple can keep going as it is. It's cash cow might die someday but by then it will have so much money they would have a century's worth of cash for R&D to find a new niche.
One thing bodes well for Google so far, Apple has struggled getting any kind of traction in the Online space. It's early for iCloud but it's still far from the competition.
I wrote something between those lines 10 days ago (http://news.ycombinator.com/item?id=3463957), so, I can understand Google's interest on having it's own mobile OS. But that's not a reason for them to start a war that is not on their best interest. Apple wins having Android around, so they can escape of being accused of monopolistic practices (e.g. no other Browser banned from their OS? Remember Microsoft?), Google wins, by having another channel of distribution for their ads.
And both of them complement each other nicely.
One thing bodes well for Google so far, Apple has struggled getting any kind of traction in the Online space. It's early for iCloud but it's still far from the competition.