Maybe it is too late now and you have already found a solution. But I will post anyhow for future users.
Using MQTT brokers like VerneMQ, Mosquitto, EMQTT you can send and receive push data to and from any server to a client and vice versa. MQTT brokers have topics which allow you to get really specific with whom to send notifications. And all this happens in real-time, and in battery friendly way. In case of poor connection or ungraceful network disconnection the messages are retained which are automagically resent to correct user when they come back online.
As a very simple setup, you can pick up one MQTT broker(say VerneMQ or any other) and use Eclipse Paho Android Service in your Android app and get started with sending push notifications, messages or data in any form.
