un-break ThemeData equality (#154695)

This PR is _almost_ able to close issue #89127.

Sadly, no `InheritedModel` or custom `RenderObject`s today; instead the [WidgetState operators](https://main-api.flutter.dev/flutter/widgets/WidgetStateOperators.html) have been restructured to support equality checks.

`WidgetStateProperty.fromMap()` is now capable of accurate equality checks, and all of the `.styleFrom()` methods have been refactored to use that constructor.

(Equality checks are still broken for `WidgetStateProperty.resolveWith()`, and any other non-`const` objects that implement the interface.)

<br><br>

credit for this idea goes to @justinmc: https://github.com/flutter/flutter/issues/89127#issuecomment-2313187703

examples/api/lib/material/input_decorator/input_decoration.widget_state.0.dart

@@ -30,8 +30,8 @@ class MaterialStateExample extends StatelessWidget {
   Widget build(BuildContext context) {
     return TextFormField(
       initialValue: 'abc',
-      decoration: InputDecoration(
-        prefixIcon: const Icon(Icons.person),
+      decoration: const InputDecoration(
+        prefixIcon: Icon(Icons.person),
         prefixIconColor: WidgetStateColor.fromMap(
           <WidgetStatesConstraint, Color>{
             WidgetState.focused: Colors.green,

examples/api/lib/material/input_decorator/input_decoration.widget_state.1.dart

@@ -32,7 +32,7 @@ class MaterialStateExample extends StatelessWidget {
     return Theme(
       data: themeData.copyWith(
         inputDecorationTheme: themeData.inputDecorationTheme.copyWith(
-          prefixIconColor: WidgetStateColor.fromMap(
+          prefixIconColor: const WidgetStateColor.fromMap(
             <WidgetStatesConstraint, Color>{
               WidgetState.error:   Colors.red,
               WidgetState.focused: Colors.blue,

examples/api/lib/material/list_tile/list_tile.3.dart

@@ -45,7 +45,7 @@ class _ListTileExampleState extends State<ListTileExample> {
               _selected = !_selected;
             });
           },
-          iconColor: WidgetStateColor.fromMap(<WidgetStatesConstraint, Color>{
+          iconColor: const WidgetStateColor.fromMap(<WidgetStatesConstraint, Color>{
             WidgetState.disabled: Colors.red,
             WidgetState.selected: Colors.green,
             WidgetState.any:      Colors.black,