Add Opt-In Material Design 3 Support for Android

Summary

Add Material Design 3 (M3) support to .NET MAUI Android as an opt-in feature, allowing developers to modernize their app's appearance while maintaining backward compatibility with existing Material 2 applications.

Problem

Direct migration to Material 3 would be a breaking change because visual appearance, color schemes, and component behaviors would change for all existing apps.

Solution

Provide M3 as an opt-in feature via MSBuild property:

<PropertyGroup>
  <UseMaterial3>true</UseMaterial3>
</PropertyGroup>

Key Deliverables

• Build Property - UseMaterial3 MSBuild property to enable M3 at build time
• Dual Theme System - Maintain both M2 (default) and M3 themes simultaneously
• Conditional Widget Selection - Use M3 widgets (MaterialSwitch, etc.) when enabled
• Resource Files - styles-material3.xml and colors-material3.xml with M3 tokens

Implementation Phases

Breaking Change Analysis

None - M2 remains default, opt-in only, fully backward compatible.

Begin by locating the Android MSBuild property handling, theme resources, and control handlers referenced by the issue. Review how the existing Material 2 theme and widgets are selected, then verify the UseMaterial3 property, styles-material3.xml, colors-material3.xml, and listed control updates. Done means M2 remains the default while an opt-in build uses M3 resources and widgets without breaking existing apps.