Problem of displaying a setText

I'm creating an application, which starts on a screen, and automatically (within "n" seconds), opens another activity.

I then decided to perform load-loading tests, but I came across the following situation: It included two TextViews, and one was to display changes of a value that decreases over n seconds. I can make the normal print in Log.e, but TextView does not change.

Can anyone tell me the reason?

Java code:

package luiscarlos.tccluis;

import android.annotation.SuppressLint;
import android.app.ProgressDialog;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class TelaInicial extends AppCompatActivity {

    int n=6; // tempo maximo de permanencia da tela
    private TextView cont;
    //private ProgressDialog progresso;
    int contador = 0; //contador de tempo
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_tela_inicial);
        //progresso = new ProgressDialog(this);
        cont = (TextView) findViewById(R.id.contador);
        ContadorDeTempo();
    }

    public void ContadorDeTempo()
    {
        //progresso.setMessage(a);
        //progresso.show();
        setContentView(R.layout.activity_tela_inicial); //recortado do metodo acima. Ajuste de conteudo para a tela inicial
        new Thread(new Runnable() {
            @Override
            public void run() {
                contador++;
                try
                {
                    while(contador == 1 || contador <= n)
                    {
                        Thread.sleep(1000);
                        contador++;
                        String a = Integer.toString(n-contador);
                        cont.setText(a);  // aonde deveria setar o valor de "a"

                        Log.e("Contador = ", Integer.toString(contador));
                        Log.e("contadora = ", a);

                    }
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
                if (contador == n+1)
                {
                    Intent intent = new Intent(TelaInicial.this, TelaLogin.class); // chamada da tela seguinte
                    startActivity(intent);
                    contador++;
                }
            }
        }).start(); // inserido para iniciar a troca de activities
    }


    @Override
    protected void onPause() {
        // TODO Auto-generated method stub
        super.onPause();
        finish(); // evita que essa activity seja chamada novamente
    }
}

xml code.

<?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="luiscarlos.tccluis.TelaInicial">


    <TextView
        android:id="@+id/textView22"
        android:layout_width="181dp"
        android:layout_height="50dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:gravity="center_vertical|center_horizontal"
        android:text="Logotipo"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:layout_editor_absoluteY="215dp" />

    <TextView
        android:id="@+id/contador"
        android:layout_width="98dp"
        android:layout_height="41dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="16dp"
        android:gravity="center"
        android:text="mudar"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView22"
        app:layout_constraintVertical_bias="0.763" />

</android.support.constraint.ConstraintLayout>