Add MonochromeFile support for Android adaptive icons

[jfversluis]

Description

Fixes #22543

Android 13+ supports themed icons with three layers: background, foreground, and monochrome. MAUI currently hardcodes the monochrome layer to always reference the foreground drawable. This causes incorrect rendering when the foreground contains multiple colors — themed icons display the full-color foreground instead of a proper monochrome silhouette.

This PR adds a new MonochromeFile metadata property for MauiIcon, allowing developers to provide a dedicated monochrome image:

<MauiIcon Include="Resources/AppIcon/icon_bg.svg"
          ForegroundFile="Resources/AppIcon/icon_fg.svg"
          MonochromeFile="Resources/AppIcon/icon_mono.svg" />

Behavior

MonochromeFile specified?	Monochrome layer	Behavior
Yes	@mipmap/{name}_monochrome	Separate monochrome PNG generated at each density
No	@mipmap/{name}_foreground	Backwards compatible — existing behavior preserved

Changes

• ResizeImageInfo.cs — Added MonochromeFilename and MonochromeIsVector properties; parse MonochromeFile metadata from ITaskItem
• AndroidAdaptiveIconGenerator.cs — Added ProcessMonochrome() method that generates monochrome PNGs at each density; two XML templates (with/without monochrome); removed stale XML caching that prevented incremental updates
• ResizetizeImagesTests.cs — 3 new tests:
  • AppIconWithMonochromeFileGeneratesMonochromeLayer — verifies monochrome PNGs and XML reference
  • AppIconWithoutMonochromeFileFallsBackToForeground — verifies backwards compatibility
  • AppIconMonochromeLayerHasCorrectSize — verifies density-correct sizing

Testing

• 571 Resizetizer unit tests pass (3 new + 568 existing, 0 regressions)
• 20 pre-existing failures in SkiaSharpAppIconToolsTests are unrelated

[github-actions]

🚀 Dogfood this PR with:

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Or

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[Copilot]

Pull request overview

Adds opt-in support for Android adaptive icon monochrome layers in the Resizetizer pipeline, allowing MAUI apps to provide a dedicated monochrome source image for Android 13+ themed icons while preserving existing behavior when not provided.

Changes:

• Parse new MonochromeFile metadata into ResizeImageInfo and expose vector detection for it.
• Generate {name}_monochrome.png at all app icon part densities when MonochromeFile is provided, and emit adaptive icon XML referencing that layer.
• Add unit tests validating monochrome generation, backward-compatible fallback behavior, and expected output sizing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/SingleProject/Resizetizer/src/ResizeImageInfo.cs	Adds MonochromeFilename/MonochromeIsVector and parses MonochromeFile item metadata.
src/SingleProject/Resizetizer/src/AndroidAdaptiveIconGenerator.cs	Generates monochrome app icon part PNGs and updates adaptive icon XML to reference either monochrome or foreground.
src/SingleProject/Resizetizer/test/UnitTests/ResizetizeImagesTests.cs	Adds tests for monochrome layer generation, fallback behavior, and size correctness.

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[dotnet-policy-service]

🚨 API change(s) detected @davidortinau FYI

[jfversluis]

Good catch — replaced the unconditional File.WriteAllText with a WriteFileIfChanged helper that reads existing content first and only writes when it differs. This preserves incremental build timestamps while still picking up changes when MonochromeFile is added/removed. See c0fe91e.