Replace deprecated withOpacity in radio.1.dart example (flutter#177606)

Original PR that was intended to fix all examples:  
- flutter#177205  

It was later split into smaller PRs due to a major misunderstanding and
test failures caused by color values not matching exactly.

Similar PRs:  
- flutter#177374  
- flutter#177490  
- flutter#177540  
- flutter#177541  
- flutter#177542

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Author: AbdeMohlbi

examples/api/lib/material/radio/radio.1.dart

@@ -64,9 +64,9 @@ class _RadioExampleState extends State<RadioExample> {
               value: RadioType.backgroundColor,
               backgroundColor: WidgetStateColor.resolveWith((Set<WidgetState> states) {
                 if (states.contains(WidgetState.selected)) {
-                  return Colors.greenAccent.withOpacity(0.5);
+                  return Colors.greenAccent.withValues(alpha: 0.5);
                 } else {
-                  return Colors.grey.shade300.withOpacity(0.3);
+                  return Colors.grey.shade300.withValues(alpha: 0.3);
                 }
               }),
             ),

examples/api/test/material/radio/radio.1_test.dart

@@ -30,11 +30,11 @@ void main() {
     );
     expect(
       radioBackgroundColor.backgroundColor!.resolve(const <WidgetState>{WidgetState.selected}),
-      Colors.greenAccent.withOpacity(0.5),
+      Colors.greenAccent.withValues(alpha: 0.5),
     );
     expect(
       radioBackgroundColor.backgroundColor!.resolve(const <WidgetState>{}),
-      Colors.grey.shade300.withOpacity(0.3),
+      Colors.grey.shade300.withValues(alpha: 0.3),
     );
 
     final Radio<example.RadioType> radioSide = tester.widget<Radio<example.RadioType>>(