Gmail com caixa diálogo

Button button = (Button) findViewById(R.id.your_button_id);
button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
        builder.setTitle("Insira Assunto e Mensagem");
        final EditText subjectInput = new EditText(MainActivity.this);
        subjectInput.setHint("Assunto");
        final EditText messageInput = new EditText(MainActivity.this);
        messageInput.setHint("Mensagem");
        LinearLayout layout = new LinearLayout(MainActivity.this);
        layout.setOrientation(LinearLayout.VERTICAL);
        layout.addView(subjectInput);
        layout.addView(messageInput);
        builder.setView(layout);
        builder.setPositiveButton("Enviar", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                String subject = subjectInput.getText().toString();
                String message = messageInput.getText().toString();
                Intent emailIntent = new Intent(Intent.ACTION_SENDTO);
                emailIntent.setData(Uri.parse("mailto: endereco_de_email@gmail.com"));
                emailIntent.putExtra(Intent.EXTRA_SUBJECT, subject);
                emailIntent.putExtra(Intent.EXTRA_TEXT, message);
                startActivity(emailIntent);
            }
        });
        builder.setNegativeButton("Cancelar", null);
        builder.create().show();
    }
});