Cara Menampilkan IMEI di Android

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:id="@+id/content_home"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:orientation="vertical"
    android:gravity="center"
    android:paddingTop="@dimen/activity_vertical_margin">
    <ImageView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
android:src="@drawable/logo"/>
    <Button
        android:id="@+id/btnimei"
        android:textColor="#ffffff"
        android:background="@color/colorPrimaryDark"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="24dp"
        android:text="Klik Disini Untuk Menampilkan IMEI" />

    <TextView
        android:id="@+id/txtimei"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="170dp"
        android:gravity="center"
        android:text="Imei HP anda Akan Tampil Disini"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</LinearLayout>

package com.example.apppost;

import android.content.Context;
import android.os.Bundle;
import android.telephony.TelephonyManager;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

public class HomeShowImei extends AppCompatActivity {
    TextView txtimei;
    Button btnimei;
    String txtimei_Holder;
    TelephonyManager telephonyManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home_showimei);
//        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
//        setSupportActionBar(toolbar);
        txtimei = (TextView) findViewById(R.id.txtimei);
        btnimei = (Button) findViewById(R.id.btnimei);

        telephonyManager = (TelephonyManager) this.getSystemService(Context.TELEPHONY_SERVICE);
        btnimei.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                txtimei_Holder = telephonyManager.getDeviceId();

                txtimei.setText(txtimei_Holder);
            }
        });
    }

}