fix(material/list): align color scheme between single and multi selection list

[crisbeto]

MDC has the mdc-list-item--selected class which we were setting only on single selection mat-list-option to indicate using color that they're selected. After the recent change to indicate selection using a radio button, these look weird because the radio button inherits the list item theme palette while the background of the item stays as primary.

For reference:

These changes resolve the issue by treating single selection and multi-selection lists the same.

Note: an alternative I was considering was to change the color so that it matches the radio button color, but then the appearance of the single selection list would be inconsistent with the multi-selection list.