Handle KEYCODE_DPAD_CENTER and KEYCODE_ENTER (#140808)

https://github.com/flutter/flutter/issues/1670

Note: This PR replaces the the original PR (https://github.com/flutter/flutter/pull/138240) that got into a state where a rebase was not possible and made it difficult to determine what was causing unrelated test failures.

packages/flutter/lib/src/widgets/app.dart

@@ -1221,6 +1221,7 @@ class WidgetsApp extends StatefulWidget {
     SingleActivator(LogicalKeyboardKey.numpadEnter): ActivateIntent(),
     SingleActivator(LogicalKeyboardKey.space): ActivateIntent(),
     SingleActivator(LogicalKeyboardKey.gameButtonA): ActivateIntent(),
+    SingleActivator(LogicalKeyboardKey.select): ActivateIntent(),
 
     // Dismissal
     SingleActivator(LogicalKeyboardKey.escape): DismissIntent(),

packages/flutter/test/widgets/app_test.dart

@@ -146,6 +146,11 @@ void main() {
     await tester.sendKeyEvent(LogicalKeyboardKey.gameButtonA);
     await tester.pumpAndSettle();
     expect(checked, isTrue);
+
+    checked = false;
+    await tester.sendKeyEvent(LogicalKeyboardKey.select);
+    await tester.pumpAndSettle();
+    expect(checked, isTrue);
   // ignore: deprecated_member_use
   }, variant: KeySimulatorTransitModeVariant.all());