You actually could, hypothetically, compile Android on Android, in a sense, by chrooting into a Debian environment and compiling from there. I've done at least the chroot part (I had a Debian chroot environment with Xfce installed on my Galaxy Tab 2 for quite a while, though it was really slow and limited by the tablet's measly RAM).
The idea of running a GNU/Linux-like system on Android (reusing the device's kernel in a chroot or a similar setup) is basically the idea behind the Ubuntu for Android concept that fizzled away as Ubuntu's mobile version started to be developed: plug your phone into a dock, and the Ubuntu chroot takes over the peripheral devices and presents a desktop.
The reality is that it's running on the Android device itself, using the device's own kernel. I personally think that qualifies in a technical sense.
This is for completely superseding the previous generation. When the level of functionality of the new platform surpasses the needs of a all pure consumers, a general purpose computer will become a niche item, much like a Sony or Nintendo development system.
Rob Landley once keenly noted the signature event that must occur for one computer generation to usurp another. It must become self-hosting, i.e. it must be able to compile itself. I cannot currently compile Android on Android or iOS on iOS.