Fix InputDecorator.prefixIcon color when disabled (#149595)

## Description

This PRs makes the `InputDecoration.prefixIcon` color compliant with the M3 spec.

From M3 spec, the color should be `onSurface.withOpacity(0.38)`, see https://m3.material.io/components/text-fields/specs#e4964192-72ad-414f-85b4-4b4357abb83c

![image](https://github.com/flutter/flutter/assets/840911/298d9aaf-fcda-479a-a8dd-0ee84db98242)

## Related Issue

Fixes flutter/flutter#149411.

## Tests

Updates 2 tests.

dev/tools/gen_defaults/generated/used_tokens.csv

@@ -238,6 +238,8 @@ md.comp.filled-text-field.disabled.container.color,
 md.comp.filled-text-field.disabled.container.opacity,
 md.comp.filled-text-field.disabled.label-text.color,
 md.comp.filled-text-field.disabled.label-text.opacity,
+md.comp.filled-text-field.disabled.leading-icon.color,
+md.comp.filled-text-field.disabled.leading-icon.opacity,
 md.comp.filled-text-field.disabled.supporting-text.color,
 md.comp.filled-text-field.disabled.supporting-text.opacity,
 md.comp.filled-text-field.disabled.trailing-icon.color,

dev/tools/gen_defaults/lib/input_decorator_template.dart

@@ -97,10 +97,10 @@ class _${blockName}DefaultsM3 extends InputDecorationTheme {
   Color? get iconColor => ${componentColor("md.comp.filled-text-field.leading-icon")};
 
   @override
-  Color? get prefixIconColor => MaterialStateColor.resolveWith((Set<MaterialState> states) {${componentColor('md.comp.filled-text-field.error.leading-icon') == componentColor('md.comp.filled-text-field.leading-icon') ? '' : '''
+  Color? get prefixIconColor => MaterialStateColor.resolveWith((Set<MaterialState> states) {
     if (states.contains(MaterialState.disabled)) {
       return ${componentColor('md.comp.filled-text-field.disabled.leading-icon')};
-    }
+    }${componentColor('md.comp.filled-text-field.error.leading-icon') == componentColor('md.comp.filled-text-field.leading-icon') ? '' : '''
     if (states.contains(MaterialState.error)) {
       if (states.contains(MaterialState.hovered)) {
         return ${componentColor('md.comp.filled-text-field.error.hover.leading-icon')};

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

@@ -4736,6 +4736,9 @@ class _InputDecoratorDefaultsM3 extends InputDecorationTheme {
 
   @override
   Color? get prefixIconColor => MaterialStateColor.resolveWith((Set<MaterialState> states) {
+    if (states.contains(MaterialState.disabled)) {
+      return _colors.onSurface.withOpacity(0.38);
+    }
     return _colors.onSurfaceVariant;
   });
 

packages/flutter/test/material/input_decorator_test.dart

@@ -5443,9 +5443,7 @@ void main() {
           );
 
           final ThemeData theme = Theme.of(tester.element(findPrefixIcon()));
-          // TODO(bleroux): based on M3 spec, it should be theme.colorScheme.onSurface.withOpacity(0.38).
-          // See https://github.com/flutter/flutter/issues/149411.
-          final Color expectedColor = theme.colorScheme.onSurfaceVariant;
+          final Color expectedColor = theme.colorScheme.onSurface.withOpacity(0.38);
           expect(getPrefixIconStyle(tester).color, expectedColor);
         });
 
@@ -5921,9 +5919,7 @@ void main() {
           );
 
           final ThemeData theme = Theme.of(tester.element(findPrefixIcon()));
-          // TODO(bleroux): based on M3 spec, it should be theme.colorScheme.onSurface.withOpacity(0.38).
-          // See https://github.com/flutter/flutter/issues/149411.
-          final Color expectedColor = theme.colorScheme.onSurfaceVariant;
+          final Color expectedColor = theme.colorScheme.onSurface.withOpacity(0.38);
           expect(getPrefixIconStyle(tester).color, expectedColor);
         });