… padding and update default horizontal padding for Material 3 (#131609)

fixes [`PopupMenuItem` adds redundant padding when using `ListItem`](#128553)

### Description

- Fixed redundant `ListTile` padding when using `CheckedPopupMenuItem` or  `PopupMenuItem`  with the `ListTile` child for complex popup menu items as suggested in the docs.
- Updated default horizontal padding for popup menu items.

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

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

/// Flutter code sample for [PopupMenuButton].

// This is the type used by the popup menu below.
enum SampleItem { itemOne, itemTwo, itemThree }

void main() => runApp(const PopupMenuApp());

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

  @OverRide
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(useMaterial3: true),
      home: const PopupMenuExample(),
    );
  }
}

class PopupMenuExample extends StatefulWidget {
  const PopupMenuExample({super.key});

  @OverRide
  State<PopupMenuExample> createState() => _PopupMenuExampleState();
}

class _PopupMenuExampleState extends State<PopupMenuExample> {
  SampleItem? selectedMenu;

  @OverRide
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('PopupMenuButton')),
      body: Center(
        child: SizedBox(
          width: 150,
          height: 100,
          child: Align(
            alignment: Alignment.topLeft,
            child: PopupMenuButton<SampleItem>(
              initialValue: selectedMenu,
              // Callback that sets the selected popup menu item.
              onSelected: (SampleItem item) {
                setState(() {
                  selectedMenu = item;
                });
              },
              itemBuilder: (BuildContext context) =>
                  <PopupMenuEntry<SampleItem>>[
                const PopupMenuItem<SampleItem>(
                  value: SampleItem.itemOne,
                  child: Text('PopupMenuItem'),
                ),
                const CheckedPopupMenuItem<SampleItem>(
                  checked: true,
                  value: SampleItem.itemTwo,
                  child: Text('CheckedPopupMenuItem'),
                ),
                const PopupMenuItem<SampleItem>(
                  value: SampleItem.itemOne,
                  child: ListTile(
                    leading: Icon(Icons.cloud),
                    title: Text('ListTile'),
                    contentPadding: EdgeInsets.zero,
                    trailing: Icon(Icons.arrow_right_rounded),
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}
``` 
	
</details>

### Before

![image](https://github.com/flutter/flutter/assets/48603081/aad15ffb-ca11-4997-81d1-b46288161a4e)

- Default horizontal padding is the same as M2 (16.0), while the specs use a smaller value (12.0)
- `ListTile` nested by default in `CheckedPopupMenuItem` has redundant padding
- `PopupMenuItem` using `ListTile` as a child for complex menu items contains redundant padding.

![Screenshot 2023-07-31 at 17 17 08](https://github.com/flutter/flutter/assets/48603081/75ad1fe5-e051-42ba-badf-e20c799dee96)

### After 

- Default horizontal padding is updated for Material 3.
- `PopupMenuItem` & `CheckedPopupMenuItem` override `ListTile` padding (similar to how `ExpansionTile` overrides `ListTile` text and icon color.

![Screenshot 2023-07-31 at 17 17 25](https://github.com/flutter/flutter/assets/48603081/288cf892-5b51-4365-9855-5ef0ed2928e9)