[Java.Interop.Tools.JavaCallableWrappers] Improve missing attribute

[jonpryor]

Context: bc44f08

If "something goes wrong" and the *.jlo.xml files contain //jcw-types/type elements that are missing the //type/@package attribute, then an exception is thrown:

System.InvalidOperationException: Missing required attribute 'package'
   at Java.Interop.Tools.JavaCallableWrappers.Extensions.XmlExtensions.GetRequiredAttribute(XElement xml, String name)
   at Java.Interop.Tools.JavaCallableWrappers.Adapters.XmlImporter.ImportType(XElement xml)
   at Java.Interop.Tools.JavaCallableWrappers.Adapters.XmlImporter.Import(XElement xml)
   at Xamarin.Android.Tasks.JavaObjectsXmlFile.Import(String filename, JavaObjectsXmlFileReadType readType)
   at Xamarin.Android.Tasks.GenerateJavaCallableWrappers.GenerateWrappers(List`1 assemblies)
   at Xamarin.Android.Tasks.GenerateJavaCallableWrappers.RunTask()
   at Microsoft.Android.Build.Tasks.AndroidTask.Execute()

Which is all well and good, except I have no idea what's broken.

Something within dotnet/android should be updated so that the file causing the exception is mentioned, but we can also improve the InvalidOperationException to contain at least a "breadcrumb" for what's going wrong.

Update the InvalidOperationException to also include the contents of the XElement.ToString() which is missing the required attribute. We can then at least search file contents to find the "offending" file.

This provides a more useful exception message:

System.InvalidOperationException: Missing required attribute 'package' within element `<type name="SKCanvasElementImpl" package="" application_java_class="android.support.multidex.MultiDexApplication" mono_runtime_initialization="mono.MonoPackageManager.LoadApplication (context);" extends_type="Uno.WinUI.Graphics2DSK.SKCanvasElement" partial_assembly_qualified_name="SKCanvasElementImpl, SamplesApp">
  <constructors>
    <constructor name="SKCanvasElementImpl" method="n_.ctor:(Landroid/content/Context;)V:" jni_signature="(Landroid/content/Context;)V" managed_parameters="Android.Content.Context, Mono.Android" params="android.content.Context p0" retval="void" is_dynamically_registered="True" super_call="p0" activate_call="p0" />
  </constructors>
</type>`.
   at Java.Interop.Tools.JavaCallableWrappers.Extensions.XmlExtensions.GetRequiredAttribute(XElement xml, String name)
   at Java.Interop.Tools.JavaCallableWrappers.Adapters.XmlImporter.ImportType(XElement xml)
   at Java.Interop.Tools.JavaCallableWrappers.Adapters.XmlImporter.Import(XElement xml)
   at Xamarin.Android.Tasks.JavaObjectsXmlFile.Import(String filename, JavaObjectsXmlFileReadType readType)
   at Xamarin.Android.Tasks.GenerateJavaCallableWrappers.GenerateWrappers(List`1 assemblies)
   at Xamarin.Android.Tasks.GenerateJavaCallableWrappers.RunTask()
   at Microsoft.Android.Build.Tasks.AndroidTask.Execute()

Which shows two problems:

1. The package attribute is present, but
2. The package attribute is the empty string.

Which should be supported! The global package is A Thing That Exists™; it should be supported! (Has been supported?!)

Update XmlImporter.ImportType() so that package is not required.

Update tests/Java.Interop.Tools.JavaCallableWrappers-Tests to add an explicit test for a type in the Java global package.