[illink] enable serialization discovery for the linker

Context: dotnet/linker#2929
Context: https://github.com/dotnet/linker/blob/6e8e139a484f74c524c821c7a4a08287d77257a4/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets#L227-L229
Fixes: #7256

Serialization heuristics in the .NET 7 linker is now opt-in.

We now need to pass a new flag to the linker by default:

    <_ExtraTrimmerArgs>--enable-serialization-discovery $(_ExtraTrimmerArgs)</_ExtraTrimmerArgs>

This might also solve project builds in `Release` mode that appear to
hang:

    Task "ILLink" (TaskId:272)

Builds appear to be running this task and never exit. Let's hope this
also solves this?

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets

@@ -15,6 +15,7 @@ This file contains the .NET 5-specific targets to customize ILLink
       DependsOnTargets="GetReferenceAssemblyPaths;_CreatePropertiesCache">
     <PropertyGroup>
       <TrimmerRemoveSymbols Condition=" '$(AndroidIncludeDebugSymbols)' != 'true' ">true</TrimmerRemoveSymbols>
+      <_ExtraTrimmerArgs Condition=" '$(_EnableSerializationDiscovery)' != 'false' ">--enable-serialization-discovery $(_ExtraTrimmerArgs)</_ExtraTrimmerArgs>
       <!--
         Used for the <ILLink DumpDependencies="$(_TrimmerDumpDependencies)" /> value:
         https://github.com/dotnet/sdk/blob/a5393731b5b7b225692fff121f747fbbc9e8b140/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.ILLink.targets#L150