[bug] DropdownMenu that is open does not close upon a system back gesture

### Steps to reproduce

When pressing the Android Back button while having a DropdownMenu open, the app is closed, rather than closing the dropdown Menu. This is in contrast to the Material 2 version, DropdownButton, where when open, if the Android Back button is opened, the dropdown menu is closed.

I have gone through the material 3 spec for the dropdown, and it haven't found that the dropdown shouldn't be closed upon doing the system back gesture.

Note that in the video attached, I am using an emulator for recording purposes. This is also happening on actual devices.

To replicate

1. Tap on the DropdownMenu to open
2. Tap on the Android Back button

### Expected results

When the user taps the android back button, the dropdown menu should be closed, rather than closing the entire app / screen.

This is to be consistent with the functionality of the DropdownButton widget, which closes the Dropdown upon pressing the android back button.

### Actual results

- If Dropdown Menu is opened at the base route, pressing android back button closes the app (as in video)

- If DropdownMenu is open while on secondary route, the route is popped (Similar to calling Navigator.pop)

- If DropdownMenu is open while in a Bottom Sheet, pressing the Android Back button closes both the menu and bottom sheet

### Code sample

Code sample

```dart
import 'package:flutter/material.dart';

void main() {
runApp(const MyApp());
}

class MyApp extends StatelessWidget {
const MyApp({super.key});

@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
useMaterial3: true,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}

class MyHomePage extends StatefulWidget {
const MyHomePage({super.key, required this.title});

final String title;

@override
State createState() => _MyHomePageState();
}

class _MyHomePageState extends State {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
backgroundColor: Theme.of(context).colorScheme.inversePrimary,
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
DropdownButton(
items: [
DropdownMenuItem(child: Text('Dropdown button 1'), value: 1),
DropdownMenuItem(child: Text('Dropdown button 2'), value: 2),
DropdownMenuItem(child: Text('Dropdown button 3'), value: 3),
],
onChanged: (value) {},
),
const SizedBox(
height: 24,
),
DropdownMenu(
enableSearch: false,
label: const Text('This is a dropdown menu'),
width: MediaQuery.of(context).size.width - 32,
dropdownMenuEntries: const [
DropdownMenuEntry(
value: 1,
label: 'Dropdown menu 1',
),
DropdownMenuEntry(
value: 2,
label: 'Dropdown menu 2',
),
DropdownMenuEntry(
value: 3,
label: 'Dropdown menu 3',
),
],
),
],
),
),
);
}
}

```

### Screenshots or Video

Screenshots / Video demonstration

https://github.com/flutter/flutter/assets/7465421/e184494b-b373-4fc5-be29-8a49b272091b

### Logs

_No response_

### Flutter Doctor output

Doctor output

```console
[✓] Flutter (Channel stable, 3.22.2, on macOS 14.5 23F79 darwin-arm64, locale
en-MT)
• Flutter version 3.22.2 on channel stable at
/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 761747bfc5 (3 weeks ago), 2024-06-05 22:15:13 +0200
• Engine revision edd8546116
• Dart version 3.4.3
• DevTools version 2.34.3

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
• Android SDK at /Android/sdk
• Platform android-34, build-tools 34.0.0
• Java binary at: /Applications/Android
Studio.app/Contents/jbr/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build
17.0.10+0-17.0.10b1087.21-11572160)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.2)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 15C500b
• CocoaPods version 1.15.0

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2023.3)
• 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
17.0.10+0-17.0.10b1087.21-11572160)

[✓] Connected device (5 available)
• sdk gphone64 arm64 (mobile) • emulator-5554 •
android-arm64 • Android 14 (API 34) (emulator)
• iPhone 13 (mobile) • •
ios • com.apple.CoreSimulator.SimRuntime.iOS-17-2 (simulator)
• macOS (desktop) • macos •
darwin-arm64 • macOS 14.5 23F79 darwin-arm64
• Mac Designed for iPad (desktop) • mac-designed-for-ipad •
darwin • macOS 14.5 23F79 darwin-arm64
• Chrome (web) • chrome •
web-javascript • Google Chrome 126.0.6478.114

[✓] Network resources
• All expected network resources are available.

• No issues found!
```