fix(accordion): correctly sets the expanded item in single mode

[YonatanKra]

Pull Request

📖 Description

Two use cases solved here:

1. Focusing on a non expanded item and then triggering a change in the items causes the focused item to expand
2. Adding items with expanded attribute are not respected and the first item was always expanded

🎫 Issues

Fixes #6618

👩‍💻 Reviewer Notes

📑 Test Plan

See the tests that were added

✅ Checklist

General

• [ x ] I have included a change request file using $ yarn change
• [ x ] I have added tests for my changes.
• [ x ] I have tested my changes.
• I have updated the project documentation to reflect my changes.
• [ x ] I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• [ x ] I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

[janechu]

Looks like we need to add a change file, can you run yarn change to generate one?

[YonatanKra]

Looks like we need to add a change file, can you run yarn change to generate one?

Done @janechu

[janechu]

Looks like we need to add a change file, can you run yarn change to generate one?

Done @janechu

I believe you might need to run it again, it should generate a change file for @microsoft/fast-foundation.