fix(avatar): fix avatar disabled for icon content (#30820)

Issue number: internal

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## What is the current behavior?
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- When Avatar has an icon as content and is disabled, the icon is above
the disabled layer.

## What is the new behavior?
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this PR. -->
- add avatar with icon on disabled state page;
- add avatar with icon on snapshots tests;
- change disable state to be the :after pseudo element avatar content;


## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

[state](https://ionic-framework-git-rou-12319-ionic1.vercel.app/src/components/avatar/test/states?ionic:theme=ionic)

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Co-authored-by: ionitron <hi@ionicframework.com>