fix(material/list): align color scheme between single and multi selection list (#26074)

MDC has the `mdc-list-item--selected` class which we were setting only on single selection `mat-list-option` to indicate using color that they're selected. After the recent change to indicate selection using a radio button, these look weird because the radio button inherits the list item theme palette while the background of the item stays as `primary`.

These changes resolve the issue by treating single selection and multi-selection lists the same.

**Note:** an alternative I was considering was to change the color so that it matches the radio button color, but then the appearance of the single selection list would be inconsistent with the multi-selection list.

Author: crisbeto

src/material/list/_interactive-list-theme.scss

@@ -1,13 +1,11 @@
 @use 'sass:map';
 @use '@material/ripple' as mdc-ripple;
-@use '../core/theming/theming';
 
 // Mixin that provides colors for the various states of an interactive list-item. MDC
 // has integrated styles for these states but relies on their complex ripples for it.
 @mixin private-interactive-list-item-state-colors($config) {
   $is-dark-theme: map.get($config, is-dark);
   $active-base-color: if($is-dark-theme, white, black);
-  $selected-color: theming.get-color-from-palette(map.get($config, primary));
 
   .mat-mdc-list-item-interactive {
     &::before {
@@ -18,21 +16,6 @@
       opacity: mdc-ripple.states-opacity($active-base-color, hover);
     }
 
-    &.mdc-list-item--selected {
-      &::before {
-        background: $selected-color;
-        opacity: mdc-ripple.states-opacity($selected-color, selected);
-      }
-
-      &:not(:focus):not(.mdc-list-item--disabled):hover::before {
-        // The hover and selected opacities need to be combined to match with what the MDC
-        // ripple state would render. More details here:
-        // https://github.com/material-components/material-components-web/blob/348665978ce73694ad4518626dd70cdf5b984113/packages/mdc-ripple/_ripple-theme.scss#L450.
-        opacity: mdc-ripple.states-opacity($selected-color, hover) +
-                 mdc-ripple.states-opacity($selected-color, selected);
-      }
-    }
-
     &:focus::before {
       opacity: mdc-ripple.states-opacity($active-base-color, focus);
     }

src/material/list/list-option.ts

@@ -64,9 +64,6 @@ export interface SelectionList extends MatListBase {
   host: {
     'class': 'mat-mdc-list-item mat-mdc-list-option mdc-list-item',
     'role': 'option',
-    // As per MDC, only list items in single selection mode should receive the `--selected`
-    // class. For multi selection, the checkbox is used as indicator.
-    '[class.mdc-list-item--selected]': 'selected && !_selectionList.multiple',
     // Based on the checkbox/radio position and whether there are icons or avatars, we apply MDC's
     // list-item `--leading` and `--trailing` classes.
     '[class.mdc-list-item--with-leading-avatar]': '_hasProjected("avatars", "before")',

src/material/list/selection-list.spec.ts

@@ -1041,16 +1041,14 @@ describe('MDC-based MatSelectionList without forms', () => {
       fixture.detectChanges();
 
       expect(selectList.selected).toEqual([testListItem1]);
-      expect(listOptions[1].nativeElement.classList.contains('mdc-list-item--selected')).toBe(true);
+      expect(listOptions[1].nativeElement.getAttribute('aria-selected')).toBe('true');
 
       dispatchMouseEvent(testListItem2._hostElement, 'click');
       fixture.detectChanges();
 
       expect(selectList.selected).toEqual([testListItem2]);
-      expect(listOptions[1].nativeElement.classList.contains('mdc-list-item--selected')).toBe(
-        false,
-      );
-      expect(listOptions[2].nativeElement.classList.contains('mdc-list-item--selected')).toBe(true);
+      expect(listOptions[1].nativeElement.getAttribute('aria-selected')).toBe('false');
+      expect(listOptions[2].nativeElement.getAttribute('aria-selected')).toBe('true');
     });
 
     it('should not show check boxes', () => {