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.

从链接的 StackBlitz 复现开始，并在完成步骤 1 后继续操作线性步进器。确认已完成的步骤显示的是 "done" 图标，而不是编辑图标，同时考虑 issue 中关于 hover 和 focus 行为的说明。Done 表示已完成步骤的图标符合预期的 Material Design 行为。

由索引模型根据 Issue 内容生成。