How to get rid of: "FileSystemException: Cannot open file, path = '...' (OS Error: Permission denied, errno = 13)"?

I'm trying to display an image from my phone in my Flutter app and getting following error:

Another exception was thrown: FileSystemException: Cannot open file, path = '/storage/emulated/0/Android/data/com.android.providers.media/albumthumbs/1570277797774' (OS Error: Permission denied, errno = 13)

I don't understand the Permission denied part, because I added...

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

...to my android/app/src/mainAndroidManifest.xml file, and when running the app it shows that it really does have the permissions to read files:

Also the file I'm trying to open exists and seems fine - I can open it with other apps as well: Here's my Code:

import 'dart:io';

import 'package:flutter/material.dart';

void main() => runApp(FileOps());

class FileOps extends StatefulWidget {
  _FileOpsState createState() => _FileOpsState();
}

class _FileOpsState extends State<FileOps> {
  File localFile;
  String widgetTitle = 'Show Image';

  @override
  void initState() {
    super.initState();
    localfile.then((File _localFile) {
      setState(() {
        localFile = _localFile;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: widgetTitle,
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: Text(widgetTitle),
        ),
        body: Column(
          children: <Widget>[
            Image.file(localFile),
          ],
        ),
      ),
    );
  }

  Future<File> get localfile async {
    String imgPath =
        '/storage/emulated/0/Android/data/com.android.providers.media/albumthumbs/1570277797774';
    return File(imgPath);
  }
}

My pubspec.yaml:

name: template_app
description: A template Flutter app for testing and minimal examples

version: 1.0.0+1

environment:
  sdk: ">=2.1.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons:

dev_dependencies:
  flutter_test:
    sdk: flutter

flutter:
  uses-material-design: true

Answers to questions in comments:

• "com.android.providers.media" is not the name of my app, idk what it is - but the image I want to open is there
• My Android Version is 8.0.0, my phone is also running EMUI 8.0.0 (idk what it is exactly, some Huawei stuff)
• I can open the file with the standard app that came with the phone called "Gallery"