bug(MatAutoComplete): mat-autocomplete doesn't always announce when the results have changed while using a screenreader

#### Reproduction

Use the first example from https://material.angular.io/components/autocomplete/examples
Using NVDA or JAWS as the screen reader

Steps to reproduce:
1. Using the first example click the input (screen reader announces 1 of 3)
2. type in 't' (screen reader announces 1 of 2)
3. type in 'th' (scren reader announces 1 of 1)
4. type in 'tw' (screen reader announces nothing)
5. type in 'o' (screen reader announces nothing)


#### Expected Behavior
I would expect the screen reader to announce for step 4 that "two 1 of 1" is the only option available
and similar behaviour with step 5 where one should be the only option.

#### Actual Behavior
For steps 4 and 5 nothing is announced by the screen reader.

#### Environment

- Angular: 11.1.0
- CDK/Material: 11.1.0
- Browser(s): Firefox, Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows
- Screenreader: JAWS, NVDA