fix(menu): Set menu-item icon color only when not set on mat-icon (an…

…gular#8614)

* fix(menu): Set menu-item icon color if not set on mat-icon

Set the theme icon color only if the `color` input property was not set on the `mat-icon` component.

Fixes angular#8594

* mat-menu-item with mat-icon[color] unit tests
Add unit tests for setting the mat-icon[color] attribute on a menu item

* Make ItemIconColorsMenu more clear
Related to angular#8594

* Fix linting issues

* Fix linting

* Removed "CSS engine tests"

src/lib/menu/_menu-theme.scss

@@ -20,7 +20,7 @@
     }
   }
 
-  .mat-menu-item .mat-icon,
+  .mat-menu-item .mat-icon:not([color]),
   .mat-menu-item-submenu-trigger::after {
     color: mat-color($foreground, 'icon');
   }