in messaging/src/main/java/com/google/firebase/quickstart/Messaging.java [117:132]
private static JsonObject buildOverrideMessage() {
JsonObject jNotificationMessage = buildNotificationMessage();
JsonObject messagePayload = jNotificationMessage.get(MESSAGE_KEY).getAsJsonObject();
messagePayload.add("android", buildAndroidOverridePayload());
JsonObject apnsPayload = new JsonObject();
apnsPayload.add("headers", buildApnsHeadersOverridePayload());
apnsPayload.add("payload", buildApsOverridePayload());
messagePayload.add("apns", apnsPayload);
jNotificationMessage.add(MESSAGE_KEY, messagePayload);
return jNotificationMessage;
}