Move toggleable to widget layer (#148272)

Currently, toggleable exists independently in the material and cupertino
libraries. Cupertino's toggleable is a minimal copy of material's, and
so it makes sense to have only one toggleable in the widgets layer.

This change makes it easier for cupertino's toggleables (switch, radio,
checkbox) to potentially have as many capabilities as material
toggleables (for example, resolving in different widget states), thus
increasing support for the cupertino library.

## 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].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] 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.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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---------

Co-authored-by: Pierre-Louis <[email protected]>

packages/flutter/lib/material.dart

@@ -188,7 +188,6 @@ export 'src/material/time_picker.dart';
 export 'src/material/time_picker_theme.dart';
 export 'src/material/toggle_buttons.dart';
 export 'src/material/toggle_buttons_theme.dart';
-export 'src/material/toggleable.dart';
 export 'src/material/tooltip.dart';
 export 'src/material/tooltip_theme.dart';
 export 'src/material/tooltip_visibility.dart';

packages/flutter/lib/src/cupertino/checkbox.dart

@@ -6,7 +6,6 @@ import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
 import 'constants.dart';
-import 'toggleable.dart';
 
 // Examples can assume:
 // bool _throwShotAway = false;

packages/flutter/lib/src/cupertino/radio.dart

@@ -6,7 +6,6 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';
-import 'toggleable.dart';
 
 // Examples can assume:
 // late BuildContext context;

packages/flutter/lib/src/material/checkbox.dart

@@ -12,7 +12,6 @@ import 'debug.dart';
 import 'material_state.dart';
 import 'theme.dart';
 import 'theme_data.dart';
-import 'toggleable.dart';
 
 // Examples can assume:
 // bool _throwShotAway = false;
@@ -444,6 +443,9 @@ class _CheckboxState extends State<Checkbox> with TickerProviderStateMixin, Togg
   @override
   bool? get value => widget.value;
 
+  @override
+  Duration? get reactionAnimationDuration => kRadialReactionDuration;
+
   MaterialStateProperty<Color?> get _widgetFillColor {
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
       if (states.contains(MaterialState.disabled)) {

packages/flutter/lib/src/material/radio.dart

@@ -13,7 +13,6 @@ import 'material_state.dart';
 import 'radio_theme.dart';
 import 'theme.dart';
 import 'theme_data.dart';
-import 'toggleable.dart';
 
 // Examples can assume:
 // late BuildContext context;
@@ -412,6 +411,9 @@ class _RadioState<T> extends State<Radio<T>> with TickerProviderStateMixin, Togg
   @override
   bool? get value => widget._selected;
 
+  @override
+  Duration? get reactionAnimationDuration => kRadialReactionDuration;
+
   MaterialStateProperty<Color?> get _widgetFillColor {
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
       if (states.contains(MaterialState.disabled)) {

packages/flutter/lib/src/material/switch.dart

@@ -18,7 +18,6 @@ import 'shadows.dart';
 import 'switch_theme.dart';
 import 'theme.dart';
 import 'theme_data.dart';
-import 'toggleable.dart';
 
 // Examples can assume:
 // bool _giveVerse = true;
@@ -746,6 +745,9 @@ class _MaterialSwitchState extends State<_MaterialSwitch> with TickerProviderSta
   @override
   bool? get value => widget.value;
 
+  @override
+  Duration? get reactionAnimationDuration => kRadialReactionDuration;
+
   void updateCurve() {
     if (Theme.of(context).useMaterial3) {
       position