AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.asuscomm.mahaljsp.lcc">

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/icon"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppThemeDialog"
        android:usesCleartextTraffic="true">
        <provider
            android:name="androidx.core.content.FileProvider"
            android:authorities="${applicationId}.fileprovider"
            android:exported="false"
            android:grantUriPermissions="true">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/provider_paths"/>
        </provider>
    </application>

MainActivity.java

package com.asuscomm.mahaljsp.lcc;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.content.FileProvider;
import android.Manifest;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Toast;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;
import java.util.HashMap;
import java.util.Map;

public class MainActivity extends AppCompatActivity {
    final int REQUEST_PERMISSION=101;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        G.context=this;
        getPermission();
    }
    public void btnUpdate_click(View view){
        /*
        Intent intent = new Intent(this, DownloadService.class);
        intent.putExtra("apkUrl", String.format("/files/android/%s", G.apk));
        startService(intent);
        */
        new Thread(()->{
            downloadFile(String.format("/files/android/%s", G.apk));
            installApk(new File(Environment.getExternalStorageDirectory().toString()+"/Download", G.apk));
        }).start();
    }
    private void init(){
    }
    private void getPermission(){
        if(Build.VERSION.SDK_INT>= Build.VERSION_CODES.M){
            if(checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
                requestPermissions(
                        new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                        REQUEST_PERMISSION);
            }
            else{
                init();
            }
        }
        else{
            init();
        }
    }
    @Override
    public void onRequestPermissionsResult(
            int requestCode,
            @NonNull String[] permissions,
            @NonNull int[] grantResults) {
        if(requestCode==REQUEST_PERMISSION){
            Map<String, Integer> perms = new HashMap<String, Integer>();
            perms.put(Manifest.permission.WRITE_EXTERNAL_STORAGE, PackageManager.PERMISSION_GRANTED);
            for (int i = 0; i < permissions.length; i++)
                perms.put(permissions[i], grantResults[i]);
            if (perms.get(Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED) {
                init();
            }
            else{
                Toast.makeText(this, "No permission", Toast.LENGTH_LONG).show();
                finish();
            }
        }
        else {
            super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
    private void installApk(File apkFile) {
        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
            Uri uri= FileProvider.getUriForFile(this, BuildConfig.APPLICATION_ID +".fileprovider", apkFile);
            intent.setDataAndType(uri, "application/vnd.android.package-archive");
        } else {
            intent.setDataAndType(Uri.fromFile(apkFile), "application/vnd.android.package-archive");
        }
        startActivity(intent);
    }
    private String downloadFile(String strUrl){
        int count;
        try {
            URL url = new URL(strUrl);
            URLConnection connection = url.openConnection();
            connection.connect();
            int lenghtOfFile = connection.getContentLength();
            // download the file
            InputStream input = new BufferedInputStream(url.openStream(),8192);
            // Output stream
            OutputStream output = new FileOutputStream(
                    String.format("%s/Download/%s", Environment.getExternalStorageDirectory().toString(),G.apk)
            );
            byte data[] = new byte[1024];
            long total = 0;
            while ((count = input.read(data)) != -1) {
                total += count;
                output.write(data, 0, count);
            }
            output.flush();
            output.close();
            input.close();
        } catch (Exception e) {
            Log.d("Thomas", "download error"+e.getMessage());
        }
        return null;
    }
}

G.java

public class G {
    public static Context context;
    //public static String apk="Lcc.apk";
    public static String apk="PatrolCar_3.0.4.apk";
}

res/provider_paths.xml

<?xml version ="1.0" encoding ="utf-8"?>
<resources>
    <paths>
        <root-path name="root" path="" />
        <files-path name="files" path="" />
        <cache-path name="cache" path="" />
        <external-path name="external" path="" />
        <external-files-path name="external_file_path" path="" />
        <external-cache-path name="external_cache_path" path="" />
    </paths>
</resources>