[material_ui] Use ColorScheme.scrim for modal barrier/scrim defaults

[byshy]

When modal barrier or drawer scrim colors are not set via an explicit parameter or component theme, fall back to ColorScheme.scrim with an opacity matching Colors.black54 instead of hardcoding Colors.black54.

This allows apps that customize ColorScheme.scrim to theme dialog, modal bottom sheet, and drawer scrims consistently, while preserving the default appearance for the standard black scrim token.

Fallback order remains:

1. Explicit parameter (barrierColor / drawerScrimColor)
2. Component theme (DialogTheme, BottomSheetTheme, DrawerTheme)
3. Theme.of(context).colorScheme.scrim.withValues(alpha: Colors.black54.a)

Ports flutter/flutter#190911
Work towards flutter/flutter#188444

Fixes flutter/flutter#190824

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[gemini-code-assist]

Code Review

This pull request updates the default fallback colors for dialog, modal bottom sheet, and drawer barriers/scrims to use ColorScheme.scrim with an opacity matching Colors.black54 when no explicit color or theme-specific color is provided. It also updates the corresponding documentation, adds a pending changelog entry, and introduces comprehensive widget tests to verify the new fallback behavior and precedence rules. There are no review comments, so I have no feedback to provide.