fix(checkbox, radio): make disabled label disabled match spec color (#4507)

Author: tinayuangao

src/lib/checkbox/_checkbox-theme.scss

@@ -63,6 +63,10 @@
         border-color: $disabled-color;
       }
     }
+
+    .mat-checkbox-label {
+      color: $disabled-color;
+    }
   }
 
   .mat-checkbox:not(.mat-checkbox-disabled) {

src/lib/radio/_radio-theme.scss

@@ -33,5 +33,9 @@
     .mat-radio-ripple .mat-ripple-element, .mat-radio-inner-circle {
       background-color: mat-color($foreground, disabled);
     }
+
+    .mat-radio-label-content {
+      color: mat-color($foreground, disabled);
+    }
   }
 }