bug(autocomplete): reset input value doesn't update the selected option

#### Reproduction

When an autocomplete control is reset, the selected option still hold the `.mat-selected` class.

https://stackblitz.com/edit/angular-ufhza2
Steps to reproduce:

1) Open the autocomplete
2) Select an entry
3) Click reset
4) Open the autocomplete again


#### Expected Behavior

`mat-selected` class should be removed when autocomplete is empty

#### Environment

- Angular: `10.2.0`
- CDK/Material: `10.2.5`

Start by reproducing the behavior in the linked StackBlitz: select an autocomplete entry, reset the control, and reopen it. Trace the autocomplete selection and reset handling to find why the selected option retains the `mat-selected` class; done means the class is removed when the input is empty, with coverage for the reported steps.

Written by the indexing model from the issue text.