fix(selection-list): proper styling not being applied when using mat-list-icon

[crisbeto]

Fixes the correct class not being applied to the mat-list-option, if it's using mat-list-icon which leads to the spacing not being correct. For reference: https://stackblitz.com/edit/angular-9tnpjv.

[ngbot]

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[jelbourn]

LGTM

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