Fix text input actions in DropdownMenu. (#177313)

Fixes https://github.com/flutter/flutter/issues/177009

### Description

- Moves `MenuButton` submit logic from `onEditingComplete` to
`onSubmitted` to allow `TextField` to handle the `textInputAction` logic
- Wraps each item into `ExcludeFocus` to enable proper
`TextInputAction.previous` handling. If we don't wrap each child in
`ExcludeFocus`, then focus will be moved to one of them, which is not
the expected behavior for `TextInputAction.previous`.

| BEFORE | AFTER |
| - | - |
| <video alt="before"
src="https://github.com/user-attachments/assets/a50d41de-7e54-409b-bf81-80dfb1db132f"
/> | <video alt="after"
src="https://github.com/user-attachments/assets/152e47e6-d774-481c-8478-af526b5f6749"
/> |

<details closed><summary>Code sample</summary>

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

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

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

  @override
  State<DropdownMenuExample> createState() => _DropdownMenuExampleState();
}

class _DropdownMenuExampleState extends State<DropdownMenuExample> {
  final FocusNode _previousFocusNode = FocusNode(debugLabel: 'previous');
  final FocusNode _textFieldFocusNode = FocusNode(debugLabel: 'textField');
  final FocusNode _nextFocusNode = FocusNode(debugLabel: 'next');

  @override
  void dispose() {
    _previousFocusNode.dispose();
    _textFieldFocusNode.dispose();
    _nextFocusNode.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(colorSchemeSeed: Colors.green),
      home: Scaffold(
        body: Center(
          child: Column(
            mainAxisSize: MainAxisSize.min,
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            spacing: 20,
            children: [
              TextField(
                focusNode: _previousFocusNode,
                textInputAction: TextInputAction.next,
                decoration: InputDecoration(
                  border: OutlineInputBorder(),
                  labelText: 'Previous TextField',
                ),
              ),
              DropdownMenu<String>(
                label: Text('Dropdown with filter cannot close keyboard'),
                initialSelection: 'green',
                focusNode: _textFieldFocusNode,
                requestFocusOnTap: true,
                showTrailingIcon: false,
                textInputAction: TextInputAction.next,
                onSelected: (String? color) {
                  print('SELECTED $color');
                },
                dropdownMenuEntries: [
                  DropdownMenuEntry(value: 'red', label: 'red'),
                  DropdownMenuEntry(value: 'green', label: 'green'),
                  DropdownMenuEntry(value: 'blue', label: 'blue'),
                ],
              ),
              TextField(
                focusNode: _nextFocusNode,
                textInputAction: TextInputAction.done,
                decoration: InputDecoration(
                  border: OutlineInputBorder(),
                  labelText: 'Next TextField',
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}
```

</details>

There is still one behavior I would like to discuss. When the
`showTrailingIcon: true` and `textInputAction: TextInputAction.previous`
are used, the focus moves not to the previous field but to the
`IconButton`. If we wrap the `IconButton` with `ExcludeFocus`, then this
is fixed, but I am not sure whether this is the correct way to proceed.

| TextInputAction.previous and no ExcludeFocus on IconButton |
TextInputAction.previous and ExcludeFocus on IconButton |
| - | - |
| <video alt="before"
src="https://github.com/user-attachments/assets/76c90dcf-3ea1-492f-8e67-7e987b08c2ff"
/> | <video alt="after"
src="https://github.com/user-attachments/assets/2a1600a2-e308-430e-a12f-acdc06cbf81c"
/> |

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

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[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
https://dart.googlesource.com/external/github.com/flutter/flutter/+/fc3c5a2c4b616ca5cf8a2ceb127c37be74629222