title: Mat-select: have it select the first mat-option
body: <p>I have many angular material selects dropdowns and their respective mat-options are updated in a a reactive way depending on the other values (in other words there is filtering of the options). It can look like that</p>
  
  <pre><code>&lt;mat-select #selects (selectionChange)='formChanges()' [placeholder]='element.label' [disabled]='false' required&gt;
    &lt;ng-container *ngFor="let opt of item.options; index as index"&gt;
    &lt;mat-option *ngIf="!videoService.filterStore[item.id] || videoService.filterStore[item.id].filter.includes(index)" [value]="opt"&gt;
      {{opt.label}}
    &lt;/mat-option&gt;
  &lt;/ng-container&gt;
  &lt;/mat-select&gt;
  </code></pre>
  
  <p>I am unhappy with one behaviour: I don't want to have the select unassigned. I want them to always pick the first option that passes the filter. </p>