AFAIK, that's not how Android works. Each apk is signed by a standalone certificate (which does not have to be signed by any CA), and the operating system will only allow an upgrade if the same certificate is used. Which means a developer must carefully guard the certificate's private key; if it's lost, the application can no longer be updated, but it must instead be released as a new application with a separate name. And since AFAIK this mechanism is part of the operating system (not the constantly-updated Google Play store), to bypass it would require a full OS update.

(This has other consequences: if a developer releases the same apk to several stores, but it's signed by different certificates on each store, a user who installed the apk from one store will not be able to upgrade it using the other store.)