(material-experimental/mdc-checkbox): add disabled class when disabled

[andrewseguin]

Currently the checkbox does not apply MDC's disabled styles since the class is not added when disabled.

  .mdc-checkbox--disabled {
    @include mdc-feature-targets($feat-structure) {
      @include mdc-checkbox--disabled_;
    }
  }

  ...

  @mixin mdc-checkbox--disabled_ {
    cursor: default;
    pointer-events: none;
  }

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