selection-list: improve a11y if list is disabled

This is an issue mainly for tracking.  Right now if the selection list is disabled, there is no way for the user to navigate to the different options. 

This way, screenreader users for example are not able to see what the individual options are. We should make them focusable, but just **prevent** toggling the selected state of the option. See https://github.com/angular/material2/pull/9963