title: mat-selection-list with search filter not keeping selections after a search
body: <p>I'm attempting to implement a <code>mat-selection-list</code> to have a list of characters--but at the top, you can search/filter characters. This is working except when you select a character, search for another character, then stop searching--your original selection is visible in the UI, but the model is cleared until you make another selection.</p>
  
  <p>Example of what I mean: <a href="https://stackblitz.com/edit/angular-yo2o9o" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-yo2o9o</a></p>
  
  <p>I can come up with a way to fix this--by manually pushing/removing from an array; but this doesn't keep the order selected in the list and that's important.</p>
  
  <p>Any ideas how this can be solved without some gross workaround?</p>