[ListItem] Expose item background color

[alexanderboswell]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

In some scenarios, when the ListItem is used inside a navigationLink, the background color is not correct. The issue is the NavigationLink wraps the content of the ListItem, so we need a way to expose out the cell background color to set it correctly for some custom implementation.

Also, ensured we set the background color for the listRow API also, so that we correctly apply color for scenarios in which the ListItem is displayed in a List and when it is not.

Binary change

N/A

Verification

Ensured the color returned from the new API matches the cell background color, making a NavigationLink be colored correctly.

The second row is a navigation link, and the background color is correct along all edges.

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)

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