Android push notifications with Firebase not turning on device screen

Question

I am trying to get a similar behavior with remote push notifications on the Android version of my app as I have in my iOS version. In iOS, when a notification arrives it shows a heads up message regardless of the state of the app or the device (more or less...) If the app is in the foreground, background, or screen turned off - it always shows a message. But in Android, I haven't been able to find a solution that works when the device screen is turned off. So my question is, how can I make a remote notification turn on the screen and show the notification as a heads up message?

I have the following implementation of the FirebaseMessagingService.

class NotificationService : FirebaseMessagingService() {


    override fun onMessageReceived(remoteMessage: RemoteMessage) {

        remoteMessage.data.let {
            it["body"]?.let { body -> sendNotification(body) }
        }

    }

    private fun sendNotification(messageBody: String) {
        val channelId = getString(R.string.default_notification_channel_id)
        val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        val requestCode = 0
        val fullScreenIntent = Intent(this, MainActivity::class.java)
        val fullScreenPendingIntent = PendingIntent.getActivity(
            this,
            requestCode,
            fullScreenIntent,
            PendingIntent.FLAG_IMMUTABLE or PendingIntent.FLAG_UPDATE_CURRENT
        )

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            val channel = NotificationChannel(
                channelId,
                "My Channel",
                NotificationManager.IMPORTANCE_HIGH,
            )
            channel.description = "Notifications."
            channel.lockscreenVisibility = Notification.VISIBILITY_PUBLIC
            channel.enableVibration(true)
            channel.vibrationPattern = longArrayOf(200, 200, 200, 200, 200)
            notificationManager.createNotificationChannel(channel)
        }
        val defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
        val notificationBuilder = NotificationCompat.Builder(this, channelId)
            .setSmallIcon(R.drawable.ic_notification)
            .setContentTitle(getString(R.string.alert_title))
            .setContentText(messageBody)
            .setAutoCancel(true)
            .setSound(defaultSoundUri)
            .setDefaults(NotificationCompat.DEFAULT_VIBRATE)
            .setPriority(NotificationCompat.PRIORITY_HIGH)
            .setOnlyAlertOnce(true)
            .setVisibility(NotificationCompat.VISIBILITY_PUBLIC)
            .setFullScreenIntent(fullScreenPendingIntent, true)

        val notificationId = 0
        notificationManager.notify(notificationId, notificationBuilder.build())
    }
}

I have these permissions in my manifest.

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.VIBRATE"/>
<uses-permission android:name="android.permission.WAKE_LOCK"/>
<uses-permission android:name="android.permission.USE_FULL_SCREEN_INTENT"/>

As well as the messaging service.

<service
    android:name=".NotificationService"
    android:exported="false">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT" />
    </intent-filter>
</service>

This works fine and shows a heads-up message when the app is in the foreground or background, but not on a locked / turned off screen. I have seen solutions like this one.

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {
    this.setTurnScreenOn(true);
} else {
    final Window window = getWindow();
    window.addFlags(WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON);
}

But since I am passing the fullScreenIntent directly to the notificationBuilder, I'm not sure where to add this code (Kotlin equivalent of course). It seems that it should belong to the specific Activity of the heads-up message. But how can I access that?

Alternatively, I guess I could build a custom notification with it's own Activity, but then I'm not sure how to pass the data from the notification to it. And I would rather use the default notification layout than have to make a custom one.

I hope there is a more straightforward way of solving this, preferably a simple method on the builder. I noticed that there is a setPublicVersion() method, but I'm not sure if this is applicable here or how I would implement this.

Any suggestions / solutions are most welcome! Cheers!