Divide that by the number of owners of each platform. There's your answer. Well, there's part of your answer. Chances are that Android owners are bimodal - some (like me) do a lot of surfing, others very little.

If Android users are bi-modal then simply factoring in the number of Android handset sales is not an accurate comparison either.

It sounds like what we really want is a histogram of web usage vs number of users :)