[Fluent2 Tokens] Moving Avatar calculated colors out of AvatarTokens

[mischreiber]

Platforms Impacted

• iOS
• macOS

Description of changes

The calculated colors used by Avatar are not really design tokens per se, so this PR moves them out of the AvatarTokens class entirely.

This change creates a new struct, Avatar.CalculatedColors, that is responsible for all the work of calculating the colors based on a set of our "standard" global colors.

Verification

Verified that there are no behavioral changes after this change.

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)

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