Switch to using Toggle in ActivityIndicatorDemoController

[mischreiber]

Platforms Impacted

• [ ] iOS
• [ ] visionOS
• [ ] macOS

Description of changes

Update the demo app to use a Toggle for the ActivityIndicator animation property.

Binary change

n/a - demo only

Verification

Ran UI test for ActivityIndicator and verified all passes.

Visual Verification

Before	After

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