ICU-22991 Reduce sizeof(Calendar) from 624 to 488 bytes

[FrankYFTang]

Reduce memory requirement for Calendar object by using bit fields

Before the PR, sizeof(Calendar) is 624 bytes, this reduce to 488 bytes

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

[FrankYFTang]

We need to talk to ICU TC about this since this PR change some protected member data and methods.

[FrankYFTang]

The changes investigated by this PR were already merged by other PR. Close this.