如何在android中的所有活动之上显示自定义提醒？

// 创建通知渠道
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
    NotificationChannel channel = new NotificationChannel("channel_id", "Channel Name", NotificationManager.IMPORTANCE_DEFAULT);
    channel.setDescription("Channel Description");
    NotificationManager notificationManager = getSystemService(NotificationManager.class);
    notificationManager.createNotificationChannel(channel);
}

// 构建通知内容
NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "channel_id")
        .setSmallIcon(R.drawable.notification_icon)
        .setContentTitle("Notification Title")
        .setContentText("Notification Content")
        .setPriority(NotificationCompat.PRIORITY_DEFAULT)
        .setAutoCancel(true);

// 设置点击行为
Intent intent = new Intent(this, MainActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
builder.setContentIntent(pendingIntent);

// 发送通知
NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);
notificationManager.notify(1, builder.build());