title: Calling disable() on FormArray child resets dirty state
body: <p>In Angular I have a form like the following</p>
  <pre><code>  const form = this.fb.group({
        id: 1,
        unique_identifier: &quot;a-b-c-d-e&quot;,
        name: &quot;foo&quot;,
        last_update: &quot;2020-07-31 12:22:22&quot;,
        items: this.fb.array([
          this.fb.group({
            name: &quot;Item 1&quot;,
            value: &quot;1&quot;,
          }),
        ]),
      });
  </code></pre>
  <p>When I call <code>form.get('items').markAsDirty()</code> the dirty property of the FormArray is true, as expected.</p>
  <pre><code>form.get('items').markAsDirty();
  
  console.log(form.get('items').dirty) // = true
  </code></pre>
  <p>When I call <code>disable()</code> or <code>enable()</code> on one of the children of the FormArray after that, the dirty property of the FormArray is false. I didn't expect that</p>
  <pre><code>form.get('items').markAsDirty();
  
  console.log(form.get('items').dirty) // = true
  
  form.get('items').at(0).get('value').disable();
  
  console.log(form.get('items').dirty) // = false
  
  </code></pre>
  <p>Does anyone know why that is? Or is this a bug in Angular?</p>
  <p>See also Stackblitz: <a href="https://stackblitz.com/edit/angular-ivy-koyisp?file=src%2Fapp%2Fapp.component.ts" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-ivy-koyisp?file=src%2Fapp%2Fapp.component.ts</a></p>