fix(list): prevent list icon shrinking (#9211)

* Since the list-item uses flexbox, it can happen that the `matListIcon` shrinks to make place for the `mat-list-text`. Similar as for the `matListAvatar` shrinking should be disabled for the `matListIcon`.

Fixes #8699

src/lib/list/list.scss

@@ -134,6 +134,7 @@ $mat-list-item-inset-divider-offset: 72px;
   }
 
   .mat-list-icon {
+    flex-shrink: 0;
     width: $icon-size;
     height: $icon-size;
     font-size: $icon-size;