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)
• 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
• FixAbstractMethodsStep

After CleanStep

• AddKeepAlivesStep - move to AssemblyModifierPipeline (Migrate AddKeepAlivesStep from ILLink custom step #10837)
• StripEmbeddedLibraries (Migrate StripEmbeddedLibraries from ILLink step #10695)
• RemoveResourceDesignerStep / GetAssembliesStep - move to MSBuild task or AssemblyModifierPipeline
• 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