I'll present the problem otherwise: Do you really think that, if someone was putting malicious code in a game, he would give it such obvious name ?
Well, I did better than you. Not much, but enough to find that billing.properties is (part of?) Google Play billing library ; perhaps the specific implementation for Unity, but my guess goes for the generic implementation since Unity already have it's own In-App Purchasing (IAP) library.
Therefore it's what permit to manage the in-game payments, if there's. Is it totally safe ? Well, not at 100%, no code is safe a 100%. But believe me, if it was possible for an app to use this library to make payment without your consent, it would have been done since years, quickly known, and quickly fixed.
Unlike other kind of vulnerability, it's not something that can pass unnoticed, since it create an entry in your bank account. Thousands of people would suddenly complain about one (or more) payment they didn't approved. Banks would quickly notice that they are all related to Google Apps, then turn to Google for explanation. Google would quickly discover that all the fraudulent payments come from the same App(s), study them, possibly with help from the major computer security companies, and find where the error lie.
Apps and smartphones never were, and will never be, really safe, but them being unsafe to this point would endanger a whole business model, as well as a whole societal habit. No one would let this happen.
