[material_ui] Respect ColorScheme.scrim in Drawer, ModalBottomSheet, and Dialog barriers

[JayeshJain25]

Drawer's scrim, showModalBottomSheet's barrier, and showDialog's barrier all hardcode Colors.black54 and never look at ColorScheme.scrim, so a custom scrim set via ColorScheme.fromSeed(..., scrim: ...) (or any ColorScheme constructor) is silently ignored.

Each of the three fallback chains now ends in colorScheme.scrim at the same alpha that Colors.black54 used, instead of the literal color. ColorScheme.scrim defaults to opaque black, so default app behavior is unchanged; a custom scrim color is now honored, matching the behavior described in the issue.

This is a port of flutter/flutter#191150, which was filed against flutter/flutter before Material/Cupertino were frozen there (per #188444). That PR is being closed in favor of this one.

Fixes #190824

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[gemini-code-assist]

Code Review

This pull request updates Drawer, showModalBottomSheet, and showDialog to respect ColorScheme.scrim as a fallback instead of defaulting directly to Colors.black54 when no custom barrier or scrim color is provided. Corresponding widget tests have been added to verify this behavior across all three components, and a pending changelog entry has been created. There are no review comments, so I have no feedback to provide.