SharedPreferences sharedPrefs = getApplicationContext().getSharedPreferences(NOME_PREFS, Context.MODE_PRIVATE);

if (!sharedPrefs.getBoolean("primeiroAcesso", false)) {
    Intent intent = new Intent(this, BoasVindasActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    intent.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION);
    startActivity(intent);
    finish();

    return;
}

SharedPreferences.Editor editor = sharedPrefs.edit();
editor.putBoolean("primeiroAcesso", true);
editor.commit();

Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
finish();