mdListItem: Secondary clickable md-icon needs focus style

[marcysutton]

The new focus styles in #2273 and #1594 require JS to set focus only on true keyboard interactions (hacking around the browser default of persisting focus on click). This behavior is not implemented on md-icon, but on md-button. To minimize duplication of code, mdList's secondary md-icon directives with ng-click on them should really be md-button.

To fix this, we can either transform md-icon into md-button behind the scenes or change the HTML API to use <md-button ng-click""><md-icon /></md-button>.