[Xamarin.Android.Build.Tasks] warn XA0149 on legacy __AndroidEnvironment__ resources

[jonathanpeppers]

Class libraries built with the legacy Xamarin.Android tooling embed
__AndroidEnvironment__* resources inside the managed assembly. The
.NET for Android replacement format places these files inside the
.aar produced by an Android class library under .net/env/.

NuGet already warns NU1703 for packages that still target the
deprecated MonoAndroid target framework (see also the breaking-change
notice in dotnet/docs#53550), but it does not call out that any
embedded environment files would be silently extracted and merged into
the application by ResolveLibraryProjectImports.

Changes

• Tasks/ResolveLibraryProjectImports.cs:
  • Refresh path: stop extracting __AndroidEnvironment__* embedded
    resources; emit XA0149 instead.
  • Cached path: drop the enumeration that re-surfaced previously
    extracted env files. We don't warn again on incremental builds —
    the warning is intended to be loud on the build that actually
    refreshes the assembly.
• New warning code XA0149 with two variants, modeled on the
  XA0141 (16 KB shared-library) warning so the message always names
  the assembly and, when available, the NuGet package id and version:
  • XA0149: Ignoring legacy Xamarin.Android environment file '{0}' from NuGet package '{1}' version '{2}' in assembly '{3}'. …
  • XA0149_Assembly: Ignoring legacy Xamarin.Android environment file '{0}' in assembly '{1}'. …
• Documentation/docs-mobile/messages/xa0149.md + index.md entry.

The new .net/env/*.env AAR layout used by .NET for Android class
libraries is untouched and still flows through @(LibraryEnvironments).

[Copilot]

Pull request overview

Adds a new build warning (XA0149) to surface and ignore legacy __AndroidEnvironment__* embedded resources from Xamarin.Android-era class libraries/NuGet packages, instead of silently extracting and merging them into the app via ResolveLibraryProjectImports.

Changes:

• Stop extracting __AndroidEnvironment__* embedded resources during ResolveLibraryProjectImports refresh; emit XA0149 (with package-aware and assembly-only variants).
• Add XA0149/XA0149_Assembly strings to Resources.resx (and generated designer).
• Add documentation page for XA0149 and link it from the message index.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tasks/ResolveLibraryProjectImports.cs	Emit XA0149 warnings for legacy embedded env resources instead of extracting them; remove cached-path env enumeration.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Add localized strings for XA0149 and XA0149_Assembly.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Generated accessors for the new XA0149 resource strings.
Documentation/docs-mobile/messages/xa0149.md	New documentation page describing XA0149, causes, and remediation.
Documentation/docs-mobile/messages/index.md	Add XA0149 entry to the message index.

Files not reviewed (1)

• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Generated file