Migrate away from ILLink custom steps

[sbomer]

Background

dotnet/runtime#107211 proposes moving ILLink custom steps out of the trimmer process to enable ILLink to be published trimmed and AOT compiled, transition from Mono.Cecil to System.Reflection.Metadata, and reduce the ILLink API surface.

This issue tracks the work in dotnet/android to migrate all custom steps registered via _TrimmerCustomSteps.

Before MarkStep

• SetupStep - eliminate (pass data via MSBuild properties) [illink] consolidate & remove hardcoded assembly names #9662
• FixLegacyResourceDesignerStep

During MarkStep

Will become obsolete with the trimmable type map

These steps do not need to be migrated:

• PreserveSubStepDispatcher / PreserveExportedTypes
• MarkJavaObjects
• PreserveJavaExceptions
• PreserveApplications
• PreserveRegistrations
• PreserveJavaInterfaces

edit: this one probably needs to be supported even with trimmable type map:

• FixAbstractMethodsStep - Move FixAbstractMethodsStep out of ILLink process #11029

After CleanStep

• AddKeepAlivesStep - move to AssemblyModifierPipeline (Move AddKeepAlives trimmer step to post-trimming task #10952)
• StripEmbeddedLibraries (Move StripEmbeddedLibraries to MSBuild task #10894)
• RemoveResourceDesignerStep / GetAssembliesStep - (Move RemoveResourceDesigner trimmer step to post-trim pipeline #10977)
• GenerateProguardConfiguration (Migrate GenerateProguardConfiguration from ILLink step to MSBuild task #10694)
• WarnOnPreserveAttribute (Migrate WarnOnPreserveAttribute from ILLink step to MSBuild task #10693)
• TypeMappingStep - will become obsolete with the trimmable type map

References

• Upstream proposal: Proposal: Move ILLink custom steps out of the trimmer process runtime#107211
• Custom step assembly: src/Microsoft.Android.Sdk.ILLink/
• Step registration: src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.LlvmIr.targets