Android - Listview personalizado solo muestra 1 objeto

package com.example.hermanos.basededatos;

import android.content.Intent;

import android.database.Cursor;

import android.database.sqlite.SQLiteDatabase;

import android.support.v7.app.AppCompatActivity;

import android.view.View;

import android.widget.ListView;

import java.lang.reflect.Array;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    ListView lv;

    ArrayList<String> lista;

    public int i;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        Datos datos[];

        datos=llenar_lista();

        lv= (ListView)findViewById(R.id.lista);

        DatosAdapter adapter = new DatosAdapter(this, R.layout.listview_item_row,datos);

        View header = (View)getLayoutInflater().inflate(R.layout.list_header_row,null);

        lv.addHeaderView(header);

        lv.setAdapter(adapter);

        //adaptador= new ArrayAdapter(this, android.R.layout.simple_list_item_1,lista);

        //lv.setAdapter(adaptador);

    }

    public void n_user(View view){

        Intent i = new Intent(this, Main2Activity.class);

        startActivity(i);

    }

    public Datos[] llenar_lista() {

        Datos datos[]=new Datos[]{};

        Basededatos n_user = new Basededatos(this, "Registro", null, 1);

        SQLiteDatabase bd = n_user.getWritableDatabase();

        String sql = "SELECT * FROM tareas";

        Cursor registros = bd.rawQuery(sql, null);

        i = 0;

        if (registros.moveToFirst()) {

            do {

                datos[i]= new Datos(registros.getString(1));

                i++;

            } while (registros.moveToNext());

        }

        return datos;

    }

}

package com.example.hermanos.basededatos;

public class Datos {

        public String tarea;

        public Datos(){

            super();

        }

        public Datos(String tarea){

            super();

            this.tarea = tarea;

        }

}

package com.example.hermanos.basededatos;

import android.app.Activity;

import android.content.Context;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.ArrayAdapter;

import android.widget.TextView;

public class DatosAdapter extends ArrayAdapter<Datos> {

    Context mycontext;

    int mylayout_resourseID;

    Datos mydata[] = null;

    public DatosAdapter(Context context,int layout_resourseID, Datos[] data){

        super(context,layout_resourseID,data);

        this.mycontext = context;

        this.mylayout_resourseID = layout_resourseID;

        this.mydata = data;

        }

    public View getView(int position, View convertView, ViewGroup parent){

        View row = convertView;

        Datosholder holder = null;

        if(row==null){

            LayoutInflater inflater = ((Activity)mycontext).getLayoutInflater();

            row = inflater.inflate(mylayout_resourseID,parent,false);

            holder = new Datosholder();

            holder.textview = (TextView)row.findViewById(R.id.txt);

            row.setTag(holder);

        }else{

            holder = (Datosholder)row.getTag();

        }

        Datos datos = mydata[position];

        holder.textview.setText(datos.tarea);

        return row;

    }

    static class Datosholder{

        TextView textview;

    }

}

package com.example.hermanos.basededatos;

import android.content.Context;

import android.database.sqlite.SQLiteDatabase;

import android.database.sqlite.SQLiteOpenHelper;

public class Basededatos extends SQLiteOpenHelper{

    public Basededatos(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {

        super(context, name, factory, version);

    }

    @Override

    public void onCreate(SQLiteDatabase Registro) {

        Registro.execSQL("create table tareas(codigo integer primary key autoincrement, tarea text)");

    }

    @Override

    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

    }

}

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:orientation="vertical" android:layout_width="match_parent"

    android:layout_height="match_parent">

    <LinearLayout

        android:layout_width="match_parent"

        android:layout_height="match_parent"

        android:orientation="horizontal">

        <CheckBox

            android:id="@+id/elr"

            android:layout_width="0dp"

            android:layout_height="wrap_content"

            android:layout_weight="0.1" />

        <TextView

            android:id="@+id/txt"

            android:layout_width="match_parent"

            android:layout_height="wrap_content"

            android:layout_weight="1"

            android:text="TextView" />

    </LinearLayout>

</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity">

    <LinearLayout

        android:layout_width="match_parent"

        android:layout_height="match_parent"

        android:orientation="vertical">

        <ListView

            android:id="@+id/lista"

            android:layout_width="match_parent"

            android:layout_height="284dp"

            android:clickable="false"

            android:footerDividersEnabled="false"

            android:headerDividersEnabled="false" />

        <Button

            android:id="@+id/button5"

            android:layout_width="match_parent"

            android:layout_height="wrap_content"

            android:onClick="n_user"

            android:text="@string/txt_n" />

    </LinearLayout>

</android.support.constraint.ConstraintLayout>

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:orientation="vertical" android:layout_width="match_parent"

    android:layout_height="match_parent">

    <TextView

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:text="@string/encabezado"

        android:background="#396640"

        android:textColor="#ffffff"

        android:gravity="center_horizontal"

        />

</LinearLayout>