WebRTC mic permission fails in Android WebView when loading index.html from assets folder (not external URL)

I'm currently learning how to build an Android WebView app that includes features like chat and WebRTC audio calls. All my files (HTML, JS, CSS) are loaded locally from the assets/ folder inside the app.

When I preview the app in a regular mobile browser (by opening the HTML file directly), everything works perfectly — the WebRTC call connects, chat messages work, etc.

However, after building the APK, the WebRTC call feature no longer works. I believe it's due to the microphone/audio permission not being granted correctly. I have already added all the necessary permissions in the AndroidManifest.xml, but I suspect that those permissions only apply to native components and not to the WebView.

Why would the WebView behave differently than a browser when it comes to accessing the microphone for WebRTC?

Here's my MainActivity.java:

import android.Manifest;
import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.webkit.WebView;
import android.webkit.WebSettings;
import android.webkit.WebViewClient;
import android.webkit.WebChromeClient;
import android.webkit.PermissionRequest;
import android.content.pm.PackageManager;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;

public class MainActivity extends Activity {

    private WebView webView;
    private static final int REQUEST_CODE = 123;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (ContextCompat.checkSelfPermission(this, Manifest.permission.RECORD_AUDIO) != PackageManager.PERMISSION_GRANTED ||
                ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED ||
                ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {

                ActivityCompat.requestPermissions(this,
                        new String[]{
                                Manifest.permission.RECORD_AUDIO,
                                Manifest.permission.READ_EXTERNAL_STORAGE,
                                Manifest.permission.WRITE_EXTERNAL_STORAGE
                        },
                        REQUEST_CODE);
            }
        }

        webView = new WebView(this);
        setContentView(webView);

        WebSettings webSettings = webView.getSettings();
        webSettings.setJavaScriptEnabled(true);
        webSettings.setAllowFileAccess(true);
        webSettings.setDomStorageEnabled(true);
        webSettings.setMediaPlaybackRequiresUserGesture(false);

        webView.setWebViewClient(new WebViewClient());

        webView.setWebChromeClient(new WebChromeClient() {
            @Override
            public void onPermissionRequest(final PermissionRequest request) {
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            for (String res : request.getResources()) {
                                if (res.equals(PermissionRequest.RESOURCE_AUDIO_CAPTURE)) {
                                    request.grant(new String[]{PermissionRequest.RESOURCE_AUDIO_CAPTURE});
                                    return;
                                }
                            }
                            request.deny();
                        }
                    });
                }
            }
        });

        webView.loadUrl("file:///android_asset/index.html");
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }
}

And here's my AndroidManifest.xml:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.tesnide"
    android:versionCode="1"
    android:versionName="1.0">

    <!-- SDK level -->
    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="27" />

    <!-- Permissions -->
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.RECORD_AUDIO" />
    <uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

        <activity android:name="com.tesnide.MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service android:name=".MyForegroundService"
            android:enabled="true"
            android:exported="false" />

    </application>

</manifest>

Any help, suggestions, or guidance would be greatly appreciated. Thank you so much in advance.