Better support no installed JDKs on macOS

[jonpryor]

What happens if there is no JDK installed on macOS?

$ mv ~/Library/Developer/Xamarin/jdk ~/Library/Developer/Xamarin/jdk.bk
$ sudo mv /Library/Java/JavaVirtualMachines /Library/Java/JavaVirtualMachines.bk

JdkInfo doesn't like this, which is to be expected, but how it
doesn't like it is...bad:

$ csharp -r:bin/Debug/Xamarin.Android.Tools.AndroidSdk.dll
csharp> Xamarin.Android.Tools.JdkInfo.GetKnownSystemJdkInfos();
Error getting SDK info:
System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
  at System.Xml.XmlTextReaderImpl.Throw (System.Exception e)
  at System.Xml.XmlTextReaderImpl.Throw (System.String res, System.String arg)
  at System.Xml.XmlTextReaderImpl.Throw (System.String res)
  at System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace ()
  at System.Xml.XmlTextReaderImpl.ParseDocumentContent ()
  at System.Xml.XmlTextReaderImpl.Read ()
  at System.Xml.XmlReader.MoveToContent ()
  at System.Xml.Linq.XElement.Load (System.Xml.XmlReader reader, System.Xml.Linq.LoadOptions options)
  at System.Xml.Linq.XElement.Parse (System.String text, System.Xml.Linq.LoadOptions options)
  at System.Xml.Linq.XElement.Parse (System.String text)
  at Xamarin.Android.Tools.JdkInfo+<GetLibexecJdkPaths>d__54.MoveNext ()
  ...

Er, what?

The problem is the output of /usr/libexec/java_home -X:

$ /usr/libexec/java_home -X
Unable to find any JVMs matching version "(null)".
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array/>
</plist>
No Java runtime present, try --request to install.

What's happening is that java_home is writing XML to stdout, and an
error message to stderr. ProcessUtils.Exec(), meanwhile, was
merging both stdout and stderr into one stream, and the
result was not valid XML, hence the XmlException.

The fix? Allow stderr to be separate from stdout, so that
JdkInfo.GetLibexecJdkPaths() can get valid XML.

Additionally, because I'm finding it increasingly annoying needing to
provide the logger parameter to AndroidSdkInfo/etc., update the
AndroidSdkInfo constructor so that logger is optional. If it
isn't provided, we'll write our log messages to System.Console.

[jonathanpeppers]

Should there also be an overload where you don’t have to pass a null logger?