MatAutocomplete does not always emit opened event

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
The `opened` event should be sent when the panel is open.

#### What is the current behavior?
When an autocomplete panel is initialized with no options, but they are added later, the `opened` event is never sent.

#### What are the steps to reproduce?
Open [https://stackblitz.com/edit/bug-autocomplete-opened](https://stackblitz.com/edit/bug-autocomplete-opened), then type something to open the autocomplete panel.

#### What is the use-case or motivation for changing an existing behavior?
The event is used to obtain a reference to the autocomplete panel’s HTML element, as soon as it is open.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
- Material 6.4.7
- Angular 6.1.0

#### Is there anything else we should know?
- See [MatAutcompleteTrigger:open](https://github.com/angular/material2/blob/2e4a51180b0074d59c1584c82fb8959f52660124/src/lib/autocomplete/autocomplete-trigger.ts#L609) the overlay is created but the event is never emitted because is no `MatOption` available.

- See [MatAutocompleteTrigger:_subscribeToClosingActions](https://github.com/angular/material2/blob/2e4a51180b0074d59c1584c82fb8959f52660124/src/lib/autocomplete/autocomplete-trigger.ts#L495), when changes are detected, the panel becomes visible but no event is emitted.

A possible fix is to emit events in [MatAutocomplete:_setVisibility](https://github.com/angular/material2/blob/2e4a51180b0074d59c1584c82fb8959f52660124/src/lib/autocomplete/autocomplete.ts#L196), but it’ll introduce a new (and possible unexpected) behavior to the component.

Empieza con MatAutocompleteTrigger:open y _subscribeToClosingActions en src/lib/autocomplete/autocomplete-trigger.ts, y después inspecciona MatAutocomplete:_setVisibility en src/lib/autocomplete/autocomplete.ts. Ejecuta la reproducción vinculada de StackBlitz y verifica que escribir después de añadir opciones emita opened cuando el panel se haga visible, sin introducir emisiones inesperadas.

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