Fix anchor and index path of ObjC popup demo

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

The Objective-C demo for PopupMenu wasn't anchoring its popup correctly. Without this change, the popup won't appear due to constraint conflicts. No idea how long this has been broken, but let's fix it.

While here, I'm also updating the starting selection index path to match the Swift demo.

Binary change

n/a - demo only change

Verification

iPad	iPhone

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow