How to show heads-up notification on Android 7 and older in Kotlin?

I am creating a messaging app and I need to show heads-up notifications on Android 7 and older up to Android 5.

I have created FirebaseMessagingService together with NotificationChannel to show notifications for Android 8 and higher and it works well with heads-up notifications. On Android 7 FirebaseMessagingService onMessageReceived method doesn't work when the app is in the background. So I've decided to use the BroadcastReceiver to show heads-up notifications and now the heads-up notification is shown on Android 7 for several seconds and then it stays in the drawer together with a normal notification. I even commented out FirebaseMessagingService but still get the ordinary notification together with the heads-up notification. I have a feeling that there is another way of implementing this.

Here is my code:

MyFirebaseMessagingService file:

class MyFirebaseMessagingService : FirebaseMessagingService() {

override fun onMessageReceived(remoteMessage: RemoteMessage) {
//      if (remoteMessage.notification !=null) {
//      showNotification(remoteMessage.notification?.title, remoteMessage.notification?.body)
//      }

}
fun showNotification(title: String?, body: String?, context: Context) {

        val intent = Intent(context, SearchActivity::class.java).apply {
            flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
        }
        val pendingIntent = PendingIntent.getActivity(context, 0, intent,
            PendingIntent.FLAG_UPDATE_CURRENT)
        val soundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
        val notificationBuilder = NotificationCompat.Builder(context, "my_channel_id_01").apply {
            setSmallIcon(R.drawable.my_friends_room_logo)
            setContentTitle(title)
            setContentText(body)
            setSound(soundUri)
            setDefaults(DEFAULT_ALL)
            setTimeoutAfter(2000)
            setPriority(PRIORITY_HIGH)
            setVibrate(LongArray(0))
        }
        val notificationManager = context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        notificationManager.notify(0, notificationBuilder.build())
    }

Calling the showNotification() from FirebaseBackgroundService file:

class FirebaseBackgroundService : BroadcastReceiver() {

    var myFirebaseMessagingService = MyFirebaseMessagingService()
    var notificationtitle: Any? = ""
    var notificationbody: Any? = ""

    override fun onReceive(context: Context, intent: Intent) {

       if (intent.extras != null) {
            for (key in intent.extras!!.keySet()) {
                val value = intent.extras!!.get(key)
                Log.e("FirebaseDataReceiver", "Key: $key Value: $value")
                if (key.equals("gcm.notification.title", ignoreCase = true) && value != null) {

                    notificationtitle = value
                }

                if (key.equals("gcm.notification.body", ignoreCase = true) && value != null) {

                    notificationbody = value
                }

            }
            myFirebaseMessagingService.showNotification(notificationtitle as String, notificationbody as String, context)
       }
    }
}

My JSON looks like this:

{
 "to" : "some-id",
  "priority":"high",

 "notification" : {
     "body" : "Body of Your Notification",
     "title": "Title of Your Notification",
     "content_available" : true,
     "sound": "sound1.mp3",

     "click_action" : "chat"
 },
"data": {
     "uid"  : "yOMX4OagvgXEl4w4l78F7SlqzKr2",
     "method" : "chat",
     "android_channel_id": "1"
   }
}