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

Angular Material Stepper examples page の2番目の「Stepper with customized states」例で、記載されている4つの手順に従って問題を再現します。初回読み込み時を含め、step がアクティブなときは常にカスタムアイコンが表示され、編集アイコンに切り替わらなければ作業は完了です。

索引モデルが issue の本文から書いたものです。