But how would you estimate the contribution that libc has to Android? Is libc entitled to 10% of Android's revenue? 20%? 100%, since it is a vital component? This is what the GP was asking.

Only google can answer that. But i think we can not say one lib make x%, it are all the libs and tools that are necessary. We don't now what e.g. google search uses under the hood, but all are needed so google can make only $1.