[illink] Move RemoveResourceDesignerStep into PostTrimmingPipeline (#11039)

Migrate RemoveResourceDesignerStep out of the ILLink custom step pipeline
and into PostTrimmingPipeline so it runs as an IAssemblyModifierPipelineStep
after trimming. This follows the same pattern as FixLegacyResourceDesignerStep.

- Add IAssemblyModifierPipelineStep to RemoveResourceDesignerStep
- Add constructor accepting assemblies list and logging delegate
- Remove GetAssembliesStep (no longer needed; assemblies passed directly)
- Remove both _TrimmerCustomSteps entries from targets
- Add AndroidLinkResources property to PostTrimmingPipeline task

## Pre-load assemblies once in PostTrimmingPipeline to avoid redundant resolver calls

Load all assemblies into a shared list upfront so that RemoveResourceDesignerStep
and the per-assembly processing loop operate on the same AssemblyDefinition instances
instead of calling resolver.GetAssembly() twice for each assembly.

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people