fix(material/list): fix up disabled list item styles (#18881)

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.

(cherry picked from commit 350ab4d)

Author: crisbeto

src/material/core/theming/_palette.scss

@@ -705,7 +705,7 @@ $dark-theme-background-palette: (
   selected-disabled-button: map.get($grey-palette, 800),
   disabled-button-toggle: black,
   unselected-chip: map.get($grey-palette, 700),
-  disabled-list-option: black,
+  disabled-list-option: rgba(white, 0.12),
   tooltip: map.get($grey-palette, 700),
 );
 

src/material/list/_list-theme.scss

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