> Never has there been a user facing operating system in the possession of 100s of millions of people, for which you can readily download and study the large majority of code running the system on these devices.

This is going to be completely off-topic, but most Android installations don't really qualify as "free" as they lack essential freedom to tinker with the code and put it back on the device. Vast majority of Android-based phones are purportedly severely tivoized, and many vendors actively fight any attempts to work around those limitations. That is, having an ability to read the source code doesn't make the software free.

So, on the contrary Android family of OSes (with the exception of AOSP and Replicant) is one of the mostly tightly locked-down computing platforms that has one of the core practices of not letting users accessing anything more than device vendor had allowed. Unsurprisingly, this is frowned upon from many zealots.