Google does have reference designs; if you use the same chips as the latest Nexus then you don't have to do any porting to get the latest Android release. This was especially apparent with Honeycomb where all the tablets used Tegra 2 (despite its tablet-unfriendly low fill rate). But unlike Microsoft's "chassis" concept, Google won't force anyone. It is open source after all. Also, Google likes to rotate their favor among all the chip and phone vendors, using a different vendor for each generation. Phone makers who are loyal to one SoC line find themselves advantaged with one release but disadvantaged with the next. As much as Android would benefit from Microsoft/Apple-style tighter control, Google seems philosophically opposed to it.