bug(stepper): customized states does not show custom icon when state becomes active

#### Reproduction

Use the second "Stepper with customized states" state example on your site which should show custom icons
[Stepper examples](https://material.angular.io/components/stepper/examples)

Steps to reproduce:
1. On initial load the edit icon is shown
2. Click the edit icon and it changes to the custom icon
3. Click Step 2 and it changes to the edit icon
4. Click Step 2 again and it changes to the custom icon


#### Expected Behavior

Always show the custom icon when step is active. Do not show the edit icon initially

#### Actual Behavior

Clicking the active icon switches the icon form edit to custom

#### Environment

Your example site

按照列出的四个步骤，在 Angular Material Stepper examples page 上的第二个“Stepper with customized states”示例中重现该问题。当 step 处于活动状态时（包括初始加载时）始终显示自定义图标，而不切换到编辑图标，即表示工作完成。

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