The Google Cloud Messaging platform for Android push notifications does not support multiple mutually untrusted servers sending push notifications to the same app (in particular, the server has to have a certain secret key associated with the app).
In order to support push notifications for arbitrary Zulip servers, we need to build (or deploy) a push notification bouncer service that can redirect push notifications from registered Zulip servers to the relevant Android clients.
https://github.com/mattermost/push-proxy might solve this problem? Needs testing.
#3092 is partial work towards implementing this.