[Java.Interop.Tools.JavaCallableWrappers] fix net8.0 targeting in XA (#1197)

Context: dotnet/android#8748
Context: 56b7eeb

dotnet/android#8748 attempts to bump xamarin-android to use
commit 56b7eeb, and fails to build:

	(CoreCompile target) ->
	Xamarin.Android.Build.Tasks\Utilities\MamJsonParser.cs(92,43): error CS0122: 'NotNullWhenAttribute' is inaccessible due to its protection level [Xamarin.Android.Build.Tasks\Xamarin.Android.Build.Tasks.csproj]
	Xamarin.Android.Build.Tasks\Utilities\MamJsonParser.cs(92,81): error CS0122: 'NotNullWhenAttribute' is inaccessible due to its protection level [Xamarin.Android.Build.Tasks\Xamarin.Android.Build.Tasks.csproj]
	Xamarin.Android.Build.Tasks\Utilities\MavenExtensions.cs(26,32): error CS0122: 'NotNullWhenAttribute' is inaccessible due to its protection level [Xamarin.Android.Build.Tasks\Xamarin.Android.Build.Tasks.csproj]

The cause of the failure is that 56955d9 updated
`Java.Interop.Tools.JavaCallableWrappers.csproj` to multitarget both
netstandard2.0 and net8.0.  This is nominally "fine", except that
`Java.Interop.Tools.JavaCallableWrappers.csproj` *also* sets
[`$(AppendTargetFrameworkToOutputPath)`][0]=false, which means that
both builds use the *same* output directory.  This means that the
netstandard2.0 and net8.0 build outputs clobber each other -- in
parallel! -- which means that the `Xamarin.Android.Build.Tasks.csproj`
build doesn't reliably use the netstandard2.0 output assembly.

Fix this scenario by no longer overriding the
`$(AppendTargetFrameworkToOutputPath)` MSBuild property, and only
setting `$(OutputPath)` to `$(ToolOutputFullPath)` for netstandard2.0
builds.

Finally, remove use of `XAConfig.props`.  This appears to be vestigial,
as we can't find any current code that would produce this file.

[0]: https://learn.microsoft.com/dotnet/core/project-sdk/msbuild-props#appendtargetframeworktooutputpath

Author: jonathanpeppers

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.csproj

@@ -1,13 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <PropertyGroup>
-    <XAConfigPath>..\..\bin\Build$(Configuration)\XAConfig.props</XAConfigPath>
-  </PropertyGroup>
-  <Import Condition="Exists ('$(XAConfigPath)')" Project="$(XAConfigPath)" />
-
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;$(DotNetTargetFramework)</TargetFrameworks>
-    <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <LangVersion>11.0</LangVersion>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
@@ -19,7 +13,8 @@
 
   <Import Project="..\..\TargetFrameworkDependentValues.props" />
 
-  <PropertyGroup>
+  <!-- Only use $(ToolOutputFullPath) for netstandard2.0 -->
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
     <OutputPath>$(ToolOutputFullPath)</OutputPath>
   </PropertyGroup>