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.

Beginne mit der verknüpften StackBlitz-Reproduktion und folge nach Abschluss von Schritt 1 dem linearen Stepper. Bestätige, dass abgeschlossene Schritte ein "done"-Symbol statt eines Bearbeiten-Symbols anzeigen, und berücksichtige dabei den Hinweis des Issues zum Verhalten bei Hover und Fokus. "Done" bedeutet, dass das Symbol des abgeschlossenen Schritts dem erwarteten Verhalten von Material Design entspricht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.