feat(avatar): use correct styles for icons in ionic theme (#29581)

Issue number: internal

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## What is the current behavior?
Avatar icon styles use `1em` width/height which makes them change size
based on the `font-size`

## What is the new behavior?
Updates the `ionic` theme only to:
- Set the `width` / `height` of icons in `ion-avatar` to use the scale
tokens based on the Figma design
- Removes the padding for avatars containing an icon
- Updates the existing screenshots to show the new icon sizes

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

## Other information

[Preview](https://ionic-framework-git-rou-10732-ionic1.vercel.app/src/components/avatar/test/size?ionic:theme=ionic)