Move width constraint for MSFNotification with flexible width from show() to a var

[huwilkes]

Platforms Impacted

• [x] iOS
• [ ] macOS

Description of changes

Adding our own width constraint was causing the Notification to stay squished if the user made the available width smaller while using attributed text, then increased it again. Allowing the user to set the only width constraint allows the Notification to resize properly.

Verification

Note: Modified the flexible width demo to have an attributed message

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)
• [x] Objective-C exposure (provide it only if needed)

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