Let cupertino & material switches move to the right state after dragging

[LongCatIsLooong]

Description

This pull request makes Cupertino/Material switches move their thumbs to the correct side after a drag gesture. The onChanged callback is called when the drag gesture ends at a position that indicates the switch should change its value (which is consistent with the onTap behavior, making them both call onChanged when the user reveals the intent to change the value. However this may be a breaking change).

For example, with the below switch:

Switch(
  value: false,
  onChanged: (bool newValue) { },
);

If the thumb is dragged to true (which is still doable), it will animate back to false when the user lets go.

In addition to that, it also moves CupertinoSwitch's gestures/animation logic to its state from the render object.

Related Issues

Fixes #46046

Tests

I added the following tests:

• "can veto switch dragging result" for both switches.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change. Does not seem to break internal tests.

[Afsar-Pasha]

The switch bounces a little bit when partially slid

Looks like its caused by changing _position's curves in the middle.
Maybe we can move

_position
  ..curve = Curves.ease
  ..reverseCurve = Curves.ease.flipped;

from _resumePositionAnimation to AnimationStatus when completed or dismissed.

[LongCatIsLooong]

@Afsar-Pasha thanks for pointing that out! I disabled curve switching for explicit animations (those not triggered by widget.value changes). Turns out there's an issue for a similar problem: #51627.

[Afsar-Pasha]

LGTM
Can you also fix material switch, I tried but it looks like the logic can't be moved from render object to state as _RenderSwitch extends RenderToggleable which contains most of the logic.

Also the current behavior is a little different RenderToggleable(or current CupertinoSwitch) calls onChanged whenever AnimationStatus.completed or AnimationStatus.dismissed(dragging switch to the end will call onChange even without lifting the finger up) but this does not.
I prefer this behavior than the old one, and we can't make material switch to behave like this while extending RenderToggleable.

[LongCatIsLooong]

I'll start working on this today.

Should be ready for review now.

[HansMuller]

LGTM

[HansMuller]

NICE

[HansMuller]

Changed to true at the end of a drag if the switch must be animated to the position indicated by the widget's value.

[HansMuller]

Don't think there's any value in caching this value