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Clarify performacne of SingleTickerProviderStateMixin vs TickerProviderStateMixin (#164870 )

Adds the following line to
[TickerProviderStateMixin](https://api.flutter.dev/flutter/widgets/TickerProviderStateMixin-mixin.html):

>Using one [TickerProviderStateMixin] twice is more efficient than two
[SingleTickerProviderStateMixin]s.

This is based on a discussion on Flutter Discord with @chunhtai. From
what I understood, creating multiple `SingleTickerProviderStateMixin`s
is more expensive because you need two different state which is
relatively costly compared to having just one. Not sure if this line
should should be qualified like "Using one [TickerProviderStateMixin]
twice is *generally* more efficient than two
[SingleTickerProviderStateMixin]s" or similar.

This change should be reviewed for correctness as I don't actually
understand enough about the difference between
`TickerProviderStateMixin` and `SingleTickerProviderStateMixin`.

Closes #164869

<details>

<summary>
Example case where this was relevant (click)
</summary>

I was avoiding using `TickerProviderStateMixin` like so:

```dart
    _animationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _updateScale(_animationController.value);
      });
   
    //could have just set `vsync` to `this` if using `TickerProviderStateMixin`
    WidgetsBinding.instance.addPostFrameCallback((_) {
      _verticalAnimationController =
          AnimationController.unbounded(vsync: _verticalController.position.context.vsync)
            ..addListener(() {
              _verticalController.jumpTo(_verticalAnimationController.value);
            });

      _horizontalAnimationController =
          AnimationController.unbounded(vsync: _horizontalController.position.context.vsync)
            ..addListener(() {
              _horizontalController.jumpTo(_horizontalAnimationController.value);
            });
    });
```

Instead of just:

```dart
    _animationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _updateScale(_animationController.value);
      });

    _verticalAnimationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _verticalController.jumpTo(_verticalAnimationController.value);
      });

    _horizontalAnimationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _horizontalController.jumpTo(_horizontalAnimationController.value);
      });
```

</details>

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[test-exempt].
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Fixes] where supported.
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