Keep webview playing audio in background

Question

I have a simple webview that open a music player from my website that works fine, but my problem is to keep the player playing in background when the phone lock screen. I am tried to use a service but whitout success maybe because of my knowledge limit.

MainActivity.java

package com.radio;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Build;
import android.os.Bundle;
import android.support.design.widget.CoordinatorLayout;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.ImageView;
import android.content.Intent;
import android.net.Uri;
import android.graphics.Color;

public class MainActivity extends AppCompatActivity {

    private Context context;
    private Activity activity;
    private CoordinatorLayout coordinatorLayout;
    private WebView webView;

    @SuppressLint({"SetJavaScriptEnabled", "ObsoleteSdkInt"})
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        context = getApplicationContext();
        activity = this;

        coordinatorLayout = findViewById(R.id.cl_webView);

        webView = findViewById(R.id.wv_nyoloWeb);

        loadWeb();

    }

    private void loadWeb() {

        webView.setWebViewClient(new WebViewClient() {

            @Override
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                super.onReceivedError(view, errorCode, description, failingUrl);
                webView.loadUrl("file:///android_asset/offline.html");
            }

        });

        String url = getResources().getString(R.string.url_player);

        webView.setBackgroundColor(Color.parseColor("#222222"));
        webView.loadUrl(url);
        webView.setWebChromeClient(new WebChromeClient());

        WebSettings settings = webView.getSettings();
        settings.setJavaScriptEnabled(true);
        settings.setDomStorageEnabled(true);
        settings.setCacheMode(WebSettings.LOAD_NO_CACHE);
    }

    protected void exitDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(activity);

        builder.setTitle("Player");
        builder.setMessage("Sair/Exit?");
        builder.setCancelable(true);

        builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                MainActivity.super.onBackPressed();
            }
        });

        builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {

            }
        });

        AlertDialog dialog = builder.create();
        dialog.show();
    }

    public void onBackPressed() {
        if (webView.canGoBack()) {
            webView.goBack();
        } else {
            exitDialog();
        }
    }
}

AndroidManifest.xml

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

    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/icone"
        android:label="@string/app_name"
        android:roundIcon="@drawable/icone"
        android:supportsRtl="true"
        android:usesCleartextTraffic="true"
        android:theme="@style/Theme.AppCompat.NoActionBar"
        android:hardwareAccelerated="true">
        <activity android:name=".MainActivity" android:launchMode="singleInstance">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        </application>

</manifest>

When the phone lock screen the audio play for 5 minutes and stop, I need to keep it playing like a music player. Thank you in advance for each help.