MdAutocompleteTrigger writeValue is asynchronous

#### Bug, feature request, or proposal:
Request for change (RFC)

#### What is the expected behavior?
The MdAutocompleteTrigger writeValue ValueAccesor method should be synchronous.

#### What is the current behavior?
The MdAutocompleteTrigger writeValue ValueAccesor method is asynchronous.

#### What are the steps to reproduce?
Simply implement the autocomplete and try to detect a change being reflected within the view upon updating the `formControl` through `setValue`.
Doing this requires a `setTimeout` coupled with wrapping the body of the `it` statement with `async(() => { ... })` when writing unit tests. If you wish to detect the change being relayed to the input `value`.

This is caused by the fact that (I assume) the writeValue wraps the `_setTriggerValue` method in a `Promise.resolve(...)`: https://github.com/angular/material2/blob/master/src/lib/autocomplete/autocomplete-trigger.ts#L179

#### What is the use-case or motivation for changing an existing behavior?
The motivation for this RFC is the fact that it complicates testing. If their are valid reasons to make the autocomplete trigger asynchronous then that's fine, but the documentation should reflect this fact.

#### Which versions of Angular, Material, OS, browsers are affected?
2.0.0-beta.2

Commencez par src/lib/autocomplete/autocomplete-trigger.ts, en particulier writeValue et l'appel Promise.resolve mentionné dans l'issue. Reproduisez le comportement en mettant à jour un formControl avec setValue et en vérifiant quand la valeur de l'input change. Le travail est considéré comme terminé lorsque le comportement synchrone demandé est résolu ou que le comportement asynchrone est explicitement documenté.

Rédigé par le modèle d'indexation à partir du texte de l'issue.