POST https://fcm.googleapis.com/v1/projects/<my_project_id>/messages:send
Authorization: Bearer <oauth token from GoogleCredential.getAccessToken()>
Content-Type: application/json; UTF-8
{
    "message": {
        "notification": {
            "title": "Sample title",
            "body": "Sample body 15:33:31"
        },
        "android": {
            "ttl": "172800s"
        },
        "topic": "MyTopic"
    }
}