Does google do anything to stop an app developer from just posting lots of 5 star reviews for themselves?
Short answer: No.

There is nothing that prevent developer from making lots of additional Google Accounts to provide 5 star ratings. Google recently made a change that require a Google+ enabled account to post reviews/star ratings on Google Play (at least using the on-device Play-app on certain phones) which is a small improvement from the past where a simple Gmail account would be enough.