[build] Produce an API 31 ref pack

Our default `net6.0-android` target framework version is using an older
API 31 reference pack that was previously published to NuGet.org.

This package will be installed on project restore, however it prevents
an "offline" Visual Studio installation from working out of the box.

Additionally, recent changes in the MSI generation tooling for .NET
workloads now produce an error when processing our WorkloadManifest.json
file.  That tooling now expects every pack dependency declared in the
manifest to exist on disk.

Update the build to produce a reference pack for the API level declared
in `$(AndroidDefaultTargetDotnetApiLevel)`, if it is not the same as
`$(AndroidLatestStableApiLevel)`.

Author: pjcollins

build-tools/create-packs/Directory.Build.targets

@@ -56,6 +56,11 @@
     <RemoveDir Directories="$(XamarinAndroidSourcePath)bin\Build$(Configuration)\nuget-unsigned" />
   </Target>
 
+  <Target Name="_CreateDefaultRefPack"
+      Condition=" '$(AndroidLatestStableApiLevel)' != '$(AndroidDefaultTargetDotnetApiLevel)' and Exists('$(XAInstallPrefix)xbuild-frameworks\Microsoft.Android\net6.0-android$(AndroidDefaultTargetDotnetApiLevel)\Mono.Android.dll') ">
+    <Exec Command="dotnet pack @(_GlobalProperties, ' ') -p:AndroidApiLevel=$(AndroidDefaultTargetDotnetApiLevel) &quot;$(MSBuildThisFileDirectory)Microsoft.Android.Ref.proj&quot;" />
+  </Target>
+
   <Target Name="_CreatePreviewPacks"
       Condition=" '$(AndroidLatestStableApiLevel)' != '$(AndroidLatestUnstableApiLevel)' and Exists('$(XAInstallPrefix)xbuild-frameworks\Microsoft.Android\net6.0-android$(AndroidLatestUnstableApiLevel)\Mono.Android.dll') ">
     <Exec Command="dotnet pack @(_GlobalProperties, ' ') -p:AndroidApiLevel=$(AndroidLatestUnstableApiLevel) -p:AndroidRID=android-arm -p:AndroidABI=armeabi-v7a-net6 &quot;$(MSBuildThisFileDirectory)Microsoft.Android.Runtime.proj&quot;" />
@@ -66,7 +71,7 @@
   </Target>
 
   <Target Name="CreateAllPacks"
-      DependsOnTargets="DeleteExtractedWorkloadPacks;_SetGlobalProperties;GetXAVersionInfo;_CleanNuGetDirectory;_CreatePreviewPacks">
+      DependsOnTargets="DeleteExtractedWorkloadPacks;_SetGlobalProperties;GetXAVersionInfo;_CleanNuGetDirectory;_CreatePreviewPacks;_CreateDefaultRefPack">
     <Exec Command="dotnet pack @(_GlobalProperties, ' ') -p:AndroidRID=android-arm -p:AndroidABI=armeabi-v7a-net6 &quot;$(MSBuildThisFileDirectory)Microsoft.Android.Runtime.proj&quot;" />
     <Exec Command="dotnet pack @(_GlobalProperties, ' ') -p:AndroidRID=android-arm64 -p:AndroidABI=arm64-v8a-net6 &quot;$(MSBuildThisFileDirectory)Microsoft.Android.Runtime.proj&quot;" />
     <Exec Command="dotnet pack @(_GlobalProperties, ' ') -p:AndroidRID=android-x86 -p:AndroidABI=x86-net6 &quot;$(MSBuildThisFileDirectory)Microsoft.Android.Runtime.proj&quot;" />

build-tools/scripts/DotNet.targets

@@ -5,6 +5,11 @@
   <Target Name="PackDotNet">
     <MSBuild Projects="$(_Root)build-tools\xa-prep-tasks\xa-prep-tasks.csproj" />
     <MSBuild Projects="$(_Root)Xamarin.Android.sln" Properties="DisableApiCompatibilityCheck=true" />
+    <MSBuild
+         Condition=" '$(AndroidLatestStableApiLevel)' != '$(AndroidDefaultTargetDotnetApiLevel)' "
+         Projects="$(_Root)src\Mono.Android\Mono.Android.csproj"
+         Properties="TargetFramework=net6.0;AndroidApiLevel=$(AndroidDefaultTargetDotnetApiLevel);AndroidPlatformId=$(AndroidDefaultTargetDotnetApiLevel);DisableApiCompatibilityCheck=true" >
+    </MSBuild>
     <MSBuild Projects="$(_Root)build-tools\create-packs\Microsoft.Android.Sdk.proj" Targets="CreateAllPacks" />
     <MSBuild Projects="$(_Root)build-tools\create-packs\Microsoft.Android.Sdk.proj" Targets="ExtractWorkloadPacks" />
     <!-- Clean up old, previously restored packages -->

src/Mono.Android/Mono.Android.csproj

@@ -386,8 +386,8 @@
   <Target Name="GetTargetPath" />
 
   <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
-  <!-- Only build the 'net6.0' version of 'Mono.Android.dll' for the latest stable Android version or higher. -->
-  <PropertyGroup Condition=" '$(TargetFramework)' != 'monoandroid10' And '$(AndroidApiLevel)' &lt; '$(AndroidLatestStableApiLevel)' ">
+  <!-- Only build the 'net6.0' version of 'Mono.Android.dll' for the default API level that is supported or higher. -->
+  <PropertyGroup Condition=" '$(TargetFramework)' != 'monoandroid10' And '$(AndroidApiLevel)' &lt; '$(AndroidDefaultTargetDotnetApiLevel)' ">
     <BuildDependsOn></BuildDependsOn>
   </PropertyGroup>
 

src/Xamarin.Android.Build.Tasks/Microsoft.NET.Sdk.Android/WorkloadManifest.in.json

@@ -43,7 +43,7 @@
     },
     "Microsoft.Android.Ref.31": {
       "kind": "framework",
-      "version": "31.0.101-preview.11.117"
+      "version": "@WORKLOAD_VERSION@"
     },
     "Microsoft.Android.Ref.32": {
       "kind": "framework",