fix(datepicker): producing wrong selector for disabled dates for themes inside a selector (#10904)

Fixes selector for the disabled dates inside the calendar being wrong, if the theme is scoped under a selector, e.g.

```scss
.dark-theme {
  @include angular-material-theme($dark-theme);
}
```

Relates to #10889.

Author: crisbeto

src/lib/datepicker/_datepicker-theme.scss

@@ -56,10 +56,10 @@ $mat-calendar-weekday-table-font-size: 11px !default;
   .mat-calendar-body-cell-content {
     color: mat-color($foreground, text);
     border-color: transparent;
+  }
 
-    .mat-calendar-body-disabled > &:not(.mat-calendar-body-selected) {
-      color: mat-color($foreground, disabled-text);
-    }
+  .mat-calendar-body-disabled > .mat-calendar-body-cell-content:not(.mat-calendar-body-selected) {
+    color: mat-color($foreground, disabled-text);
   }
 
   .mat-calendar-body-cell:not(.mat-calendar-body-disabled):hover,