Fixed Android agent to send the issued certificate's serial number to Fleet as hexadecimal (#45413)

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**Related issue:** Resolves #45405

The serial numbers are currently unused (they are only for debug), so we
are making a hard switch to hex without a migration.

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually

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## Summary by CodeRabbit

## Release Notes

* **Bug Fixes**
* Fixed Android agent certificate serial number reporting to use
hexadecimal format for proper Fleet compatibility.
* **Tests**
* Added test coverage to ensure certificate serial numbers are correctly
encoded as hexadecimal when reporting status.

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android/changes/45405-android-cert-serial-hex

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+- Fixed Android agent to send the issued certificate's serial number to Fleet as hexadecimal.