[Fluent 2 iOS] Calendar & DateTimePicker updates

[laminesm]

Platforms Impacted

• iOS
• macOS

Description of changes

This PR addresses recent design changes to DateTimePicker and CalendarView.

• CalendarViewDayCellBackgroundStyle which was removed in [Fluent 2 iOS] DateTimePicker + CalendarView update #1246 has been added back
• dateIsTodayOrLater function has been changed to dateIsInCurrentMonth that returns whether the date is in the current month or not
• Since design requested NOT to use elevated colors, we needed to specify the dark token inside the DynamicColor initializer for background colors
• As part of the Move fluentTheme from window to view #1219 change, I added the checks for whether the view is a descendent of the changing view in themeDidChange functions

Verification

The changes were tested on the demo app.

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