[autocomplete] panelClosingActions does not include escape key

[willshowell]

Bug, feature request, or proposal:

Bug

Current

panelClosingActions doesn't emit an event for when the autocomplete is closed due to the escape key

Expected

if (event.keyCode === ESCAPE && this.panelOpen) {
  this._escapeKeyStream.next();
}

// ...

get panelClosingActions(): Observable<MdOptionSelectionChange> {
  return merge(
    this.optionSelections,
    this.autocomplete._keyManager.tabOut,
    this._outsideClickStream,
    this._escapeKeyStream
  );
}

What are the steps to reproduce?

https://plnkr.co/edit/R0CPBfwzKS54o5Fd7fsx?p=preview

What is the use-case or motivation for changing an existing behavior?

#3334 (comment)

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