Implement FCM on android client
Then post request to FCM Server via the legacy HTTP protocol:
curl -X POST -H "Authorization: key=<YOUR_SERVER_KEY>" \
-H "Content-Type: application/json" \
-d '{
"data": {
"content": "You Have a Notification Message"
},
"to": "<YOUR_DEVICE_REGISTER_TOKEN>"
}' https://fcm.googleapis.com/fcm/send