ICU-22991 Remove protected direct access to fStamp

[FrankYFTang]

This PR prepare for future simplification of Calendar API. It replace the direct access to protected fStamp member data and replaced with protected method.

Checklist

• Required: Issue filed: ICU-22991
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable