MDC List: Accept multiples indexes for selectedIndex for checkbox based list.

[abhiomkar]

MDC List component has selectedIndex setter which updates the selection in a list, this is a proxy to foundation's setSelectedIndex(index) method. But this only works for single selection list and radio list. Checkbox based list is a multi-selectable list and it needs to accept multiple indexes to update selection.

Also need to add getter method for selectedIndex in MDC List component. selectedIndex: that accepts and return value type number|!Array<number>.