Fix `Slider` `onChanged` callback order & never calls `onChangeStart` on  `SliderInteraction.slideOnly` allowed interaction (#136720)

fixes [Slider will call onChanged before onChangeStart when sliding.](https://github.com/flutter/flutter/issues/136707)

This fixes a regression from https://github.com/flutter/flutter/pull/121483

### Code sample

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

```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 const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Example(),
    );
  }
}

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

  @override
  State<Example> createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  double _sliderValue = 0.5;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Slider(
          // allowedInteraction: SliderInteraction.tapAndSlide,
          // allowedInteraction: SliderInteraction.tapOnly,
          // allowedInteraction: SliderInteraction.slideOnly
          // allowedInteraction: SliderInteraction.slideThumb,
          value: _sliderValue,
          onChangeStart: (newValue) {
            print("onChangeStart ......");
          },
          onChanged: (newValue) {
            print("onChanged ......");
            setState(() {
              _sliderValue = newValue;
            });
          },
          onChangeEnd: (newValue) {
            print("onChangeEnd ......");
          },
        ),
      ),
    );
  }
}
```

</details>