[illink] Move RemoveResourceDesigner trimmer step to post-trim pipeline (#10977)

## [xabt] Move `RemoveResourceDesigner` trimmer step to PostTrimmingPipeline

Migrate RemoveResourceDesignerStep from an ILLink custom step to a
post-trimming IAssemblyModifierPipelineStep, following the same pattern
used for StripEmbeddedLibrariesStep and PostTrimmingAddKeepAlivesStep.

- Add PostTrimmingRemoveResourceDesignerStep with self-contained logic
  from LinkDesignerBase/RemoveResourceDesignerStep, using Action<string>
  for logging instead of ILLink's Context.LogMessage
- Add AndroidLinkResources property to PostTrimmingPipeline task and
  pre-load all assemblies when enabled (the step needs a two-phase scan)
- Remove RemoveResourceDesignerStep and GetAssembliesStep from ILLink
  custom steps in targets
- Remove AndroidLinkConfiguration.cs and RemoveResourceDesignerStep.cs
  from ILLink csproj (no longer needed there)
- Delete GetAssembliesStep.cs (dead code, only served RemoveResourceDesignerStep)

## [xabt] Rename PostTrimmingRemoveResourceDesignerStep to RemoveResourceDesignerStep

The old ILLink RemoveResourceDesignerStep is no longer compiled into
Xamarin.Android.Build.Tasks, so the PostTrimming prefix is unnecessary.
Also remove AndroidLinkConfiguration.cs from compilation as it was only
used by the old ILLink step.

Author: sbomer

src/Microsoft.Android.Sdk.ILLink/GetAssembliesStep.cs

@@ -19,7 +19,6 @@
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\FixAbstractMethodsStep.cs" Link="MonoDroid.Tuner\FixAbstractMethodsStep.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\FixLegacyResourceDesignerStep.cs" Link="MonoDroid.Tuner\FixLegacyResourceDesignerStep.cs" />
     <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\LinkDesignerBase.cs" Link="MonoDroid.Tuner\LinkDesignerBase.cs" />
-    <Compile Include="..\Xamarin.Android.Build.Tasks\Linker\MonoDroid.Tuner\RemoveResourceDesignerStep.cs" Link="MonoDroid.Tuner\RemoveResourceDesignerStep.cs" />
 
     <!--Other .NET for Android / Java.Interop files-->
     <Compile Include="..\..\external\Java.Interop\src\Java.Interop.Tools.Cecil\Java.Interop.Tools.Cecil\CustomAttributeProviderRocks.cs" Link="Java.Interop\CustomAttributeProviderRocks.cs" />