fix(material/list): fix up disabled list item styles

Fixes a couple of issues with the list item's disabled styles:
- In dark themes we were setting the disabled styles to black. This looks out of place
since it's not how other components style themselves under dark themes.
- The disabled styles are somewhat similar to the selected styles. These changes also
set the text color to the disabled text color to make it easier to distinguish.

Author: crisbeto

src/material/core/theming/_palette.scss

@@ -704,7 +704,7 @@ $mat-dark-theme-background: (
   selected-disabled-button: map-get($mat-grey, 800),
   disabled-button-toggle: black,
   unselected-chip: map-get($mat-grey, 700),
-  disabled-list-option: black,
+  disabled-list-option: rgba(white, 0.12),
   tooltip: map-get($mat-grey, 700),
 );
 

src/material/list/_list-theme.scss

@@ -22,10 +22,11 @@
     .mat-subheader {
       color: mat-color($foreground, secondary-text);
     }
-  }
 
-  .mat-list-item-disabled {
-    background-color: mat-color($background, disabled-list-option);
+    .mat-list-item-disabled {
+      background-color: mat-color($background, disabled-list-option);
+      color: mat-color($foreground, disabled-text);
+    }
   }
 
   .mat-list-option,