Repository: flutter
Issue: Semantics focus moves to first element after the focused widget is updated
Labels: framework,f: material design,a: accessibility,has reproducible steps,found in release: 2.10,found in release: 2.13,team-design,triaged-design

### Example code

```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _value;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
        leading: IconButton(icon: Icon(Icons.arrow_back), onPressed: () {}),
      ),
      body: Center(
        child: Padding(
            padding: EdgeInsets.symmetric(horizontal: 20),
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                DropdownButtonFormField(
                    value: _value,
                    onChanged: (value) {
                      setState(() {
                        _value = value;
                      });
                    },
                    hint: Text("Open dropdown to select an item"),
                    items: [
                      DropdownMenuItem(child: Text("Dummy 1"), value: 1),
                      DropdownMenuItem(child: Text("Dummy 2"), value: 2),
                      DropdownMenuItem(child: Text("Dummy 3"), value: 3)
                    ])
              ],
            )),
      ),
    );
  }
}

```

### Steps

- Turn on accessibility (Talkback) in Settings
- Start App
- Use accessibility navigation to select an item from Dropdown
- The accessibility focus jumps to first item on page (Back button)

### Expected results

The accessibility focus stays on focused widget

### Flutter doctor output

[✓] Flutter (Channel stable, 2.0.3, on macOS 11.1 20C69 darwin-x64, locale hu-HU)
    • Flutter version 2.0.3 at /Users/developer/Work/flutter
    • Framework revision 4d7946a68d (4 weeks ago), 2021-03-18 17:24:33 -0700
    • Engine revision 3459eb2436
    • Dart version 2.12.2

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at /Users/developer/Work/sdk
    • Platform android-30, build-tools 30.0.2
    • ANDROID_HOME = /Users/developer/Work/sdk
    • ANDROID_SDK_ROOT = /Users/developer/Work/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 12.4, Build version 12D4e
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] IntelliJ IDEA Community Edition (version 2020.2)
    • IntelliJ at /Applications/IntelliJ IDEA CE.app
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart

[✓] VS Code (version 1.52.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension can be installed from:
      🔨 https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (2 available)
    • SM G965F (mobile) • 279d5774421c7ece • android-arm64  • Android 10 (API 29)
    • Chrome (web)      • chrome           • web-javascript • Google Chrome 89.0.4389.114

• No issues found!