bug(Autocomplete): (optionActivated) not triggered when [autoActiveFirstOption] and option list changes

[zakhenry]

Reproduction

https://stackblitz.com/angular/vkkedkvndlkq

Steps to reproduce:

1. Open console
2. Click autocomplete field - see in console One is output ✅
3. Type "three" - see no option is output in console ❌
4. Type {backpace} 4x until just T remains
5. See that Two is output in the console ✅

Expected Behavior

When autoActiveFirstOption input is set, every time the list of options updates the (optionActivated) eventemitter should emit the first value in the list of options (if it changes).

Actual Behavior

When causing less values to be in the list, no events emitted, but when more items appear in the list (backspacing the search) the items were emitted.

Environment

• Angular: ^9.0.0
• CDK/Material: ^9.0.0
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): Mac

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