Linear stepper shows edit icons instead of done icons

#### Bug:
Stepper shows edit icons instead of "done" icons

#### What is the expected behavior?
Completed steps should have a "done" icon on a linear stepper.

#### What is the current behavior?
Completed steps show an edit icon

#### What are the steps to reproduce?
https://angular-19juam.stackblitz.io
1. Fill step 1
2. Click Next
3. Notice icon on label

#### What is the use-case or motivation for changing an existing behavior?
The checkmark makes it easier to understand that a step is done. Perhaps the edit icon is still useful for `:hover` and `:focus`. Or perhaps the edit icon would work better on a non-linear stepper.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Current build: 5.0.0-rc.3-05d726d

#### Is there anything else we should know?
On the issue #7260, it was marked as fixed but the stepper doesn't meet the material guidelines yet, and this is one of the reasons why.

Inizia con la riproduzione StackBlitz collegata e segui lo stepper lineare dopo aver completato il passaggio 1. Verifica che i passaggi completati mostrino un'icona "done" anziché un'icona di modifica, tenendo conto della nota dell'issue sul comportamento al passaggio del mouse e al focus. Done significa che l'icona del passaggio completato corrisponde al comportamento previsto da Material Design.

Scritto dal modello di indicizzazione a partire dal testo della issue.