Accessible checkbox

Users should be able to to interact with `material-checkbox` components using a screen reader and keyboard. These changes make those components into interactive elements by adding `tabIndex` and visual feedback on focus, the ARIA `checkbox` role and a dynamically-updated `aria-checked="[ true | false ]"` attribute.
One thing I noticed here: because the latter two components don't have `ng-model`, they are not affected by the `render` function. That means the `checkbox` role and `aria-checked` attribute aren't added to those two components until you click or use the keyboard. This is incorrect and needs fixing.