fix(material-experimental/multiple): fix disabled label style

MDC does not set the disabled text color on the label for the checkbox,
radio, or switch. This change applies it on the Angular Material side.

Author: mmalerba

src/material-experimental/mdc-checkbox/_checkbox-theme.scss

@@ -32,6 +32,7 @@
   $primary: mat.get-color-from-palette(map.get($config, primary));
   $accent: mat.get-color-from-palette(map.get($config, accent));
   $warn: mat.get-color-from-palette(map.get($config, warn));
+  $foreground: map.get($config, foreground);
 
   @include mat.private-using-mdc-theme($config) {
     .mat-mdc-checkbox {
@@ -42,6 +43,11 @@
         ),
       ));
 
+      // MDC should set the disabled color on the label, but doesn't, so we do it here instead.
+      &.mdc-checkbox--disabled label {
+        color: mat.get-color-from-palette($foreground, disabled-text);
+      }
+
       .mdc-checkbox__ripple {
         background: mdc-theme-color.prop-value(on-surface);
       }

src/material-experimental/mdc-radio/_radio-theme.scss

@@ -21,6 +21,7 @@
   $primary: mat.get-color-from-palette(map.get($config, primary));
   $accent: mat.get-color-from-palette(map.get($config, accent));
   $warn: mat.get-color-from-palette(map.get($config, warn));
+  $foreground: map.get($config, foreground);
 
   @include mat.private-using-mdc-theme($config) {
     $on-surface: rgba(mdc-theme-color.$on-surface, 0.54);
@@ -46,6 +47,11 @@
 
       --mat-mdc-radio-ripple-color: #{mdc-theme-color.prop-value(on-surface)};
 
+      // MDC should set the disabled color on the label, but doesn't, so we do it here instead.
+      .mdc-radio--disabled + label {
+        color: mat.get-color-from-palette($foreground, disabled-text);
+      }
+
       &.mat-primary {
         @include _color-palette($primary);
       }

src/material-experimental/mdc-slide-toggle/_slide-toggle-theme.scss

@@ -73,6 +73,7 @@
   $accent: mat.get-color-from-palette(map.get($config, accent));
   $warn: mat.get-color-from-palette(map.get($config, warn));
   $is-dark: map.get($config, is-dark);
+  $foreground: map.get($config, foreground);
 
   @include mat.private-using-mdc-theme($config) {
     // MDC's switch doesn't support a `color` property. We add support
@@ -81,6 +82,11 @@
       @include mdc-form-field.core-styles($query: mat.$private-mdc-theme-styles-query);
       @include mdc-switch-theme.theme(_get-theme-base-map($is-dark));
 
+      // MDC should set the disabled color on the label, but doesn't, so we do it here instead.
+      .mdc-switch--disabled + label {
+        color: mat.get-color-from-palette($foreground, disabled-text);
+      }
+
       &.mat-primary {
         @include mdc-switch-theme.theme(_get-theme-color-map($primary));
       }