Use Dev Home icon for widget screenshots

[krschau]

Summary of the pull request

We don't technically support Dev Home widgets in the Windows Widget panel, but the picker should look less bad. Having trouble taking screenshots of the picker to paste here.

References and relevant issues

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• [ ] Closes https://github.com/microsoft/devhome/issues/1991
• [ ] Tests added/passed
• [ ] Documentation updated