[Xamarin.Android.Build.Tasks] JavaTypeSystem & XML Adjuster (#6444)

Context: dotnet/java-interop@f658ab2

In commit 43243b4 we added the `$(_AndroidUseJavaLegacyResolver)`
MSBuild property to the `<BindingsGenerator/>` MSBuild task, to allow
users to use the older "API XML Adjuster" instead of the new
`Java.Interop.Tools.JavaTypeSystem.dll` infrastructure added in
dotnet/java-interop@f658ab26.

However, we call `generator` twice during the build: once for
`ApiXmlAdjuster`, and once to generate C# code, but  we only added
the the `$(_AndroidUseJavaLegacyResolver)` to the "generate C# code"
case.  Update the `_ExportJarToXml` target so that
`$(_AndroidUseJavaLegacyResolver)` is used in the `ApiXmlAdjuster`
scenario, which is where it is primarily needed.

Author: jpobst

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Bindings.ClassParse.targets

@@ -43,6 +43,7 @@ This file is only used by binding projects.
         ToolPath="$(MonoAndroidToolsDirectory)"
         ToolExe="$(BindingsGeneratorToolExe)"
         Nullable="$(Nullable)"
+        UseJavaLegacyResolver="$(_AndroidUseJavaLegacyResolver)"
     />
   </Target>