I have a history in automotive software. I have bought the Prius V for its drive-train and for lack of alternatives. I bought the i-tech version with the full set of digital features. I know that the head unit in the Prius can be updated and has necessary performance to support Android Auto. I have asked Toyota to support this and have not had any feedback. If Toyota does not offer an upgrade to the unit in the next two years that enables this, I will not buy a Toyota again. There is no reason why this cannot be done. Toyota will have difficulty marketing the Prius as a smart and sustainable vehicle when its core - the control software - is outdated and dumb and does not see attention.
Major changes to the software/firmware for a 3 year old car aren't something a profit oriented company is interested in unless mandated by law. Their developers are more focused on cars for 3 years from now. And in at least one new Toyota product, they have allowed lots more externally provided functionality. Just imagine the fun or qualifying all the phone possibilities in all the possible update conditions and app revisions.