fix(material-experimental/mdc-slider): code review changes

* increase opacity of value indicator tooltip when in dark mode
* use surface and on-surface for tick-mark-active and tick-mark-inactive disabled colors

Author: wagnermaciel

src/material-experimental/mdc-slider/_slider-theme.scss

@@ -13,10 +13,11 @@
         $is-dark: map-get($config, is-dark);
         $indicator-color: if($is-dark, white, black);
         $indicator-text-color: if($is-dark, black, white);
+        $indicator-opacity: if($is-dark, 0.9, 0.6);
 
         @include value-indicator-color(
           $color: $indicator-color,
-          $opacity: 0.6,
+          $opacity: $indicator-opacity,
           $query: $mat-theme-styles-query
         );
         @include value-indicator-text-color(
@@ -93,14 +94,14 @@
   @include tick-mark-active-color(
     $color-or-map: (
       default: $on-color,
-      disabled: $on-color,
+      disabled: surface,
     ),
     $query: $mat-theme-styles-query
   );
   @include tick-mark-inactive-color(
     $color-or-map: (
       default: $color,
-      disabled: $on-color,
+      disabled: on-surface,
     ),
     $query: $mat-theme-styles-query
   );