Add Support For Expander In FindChild

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Fixes

Addresses #472

Adds support for searching an Expander control's header. The current logic only searches the content of the Expander control, but not the header.

PR Type

What kind of change does this PR introduce?

Bugfix

What is the current behavior?

Doesn't find framework elements in the header of an Expander control.

What is the new behavior?

Finds framework elements in the header of an Expander control.

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [x] Based off latest main branch of toolkit
• [ ] Tested code with current supported SDKs
• [x] Contains NO breaking changes

Other information

No tests. I believe the Logical Tree Tests are not being run (#474).