feat(input): add styles for clear action to ionic theme (#29337)

Issue number: Internal

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## What is the new behavior?
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For the `ionic` theme: Adjusted the size of the clear button to match
design specs, and updated the behavior so the button additionally only
shows if the native input *or* any other actions within the `ion-input`
are focused. (See code comments for details.)

The actual icon used has not been modified. The ticket says to "use the
material design clear icon as the reference icon for this feature," and
we've already added a feature allowing the icon to be customized at the
app level. Let me know if the intention was to use a different default
icon with the `ionic` theme.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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