Is it possible? Sure.

Even then, not really. Not legally, anyway. Open source licences require that the user be provided with the source code (if requested) alongside the binaries. If they roll out an update to Android (to code which is under an open source licence), they have to release the code at essentially the same time. Rolling out an update and then withholding the source code for an unnecessarily long time would be against the terms of the licence.