Support for Empty namespace name for java classes without package

[gugavaro]

Generator currently generates invalid code if we try to bind a java library that contains classes/interfaces defined without/outside a java package.

The current behavior generates the following invalid code:

namespace  {

	// Metadata.xml XPath class reference: path="/api/package[@name='']/class[@name='Animal']"
	[global::Android.Runtime.Register ("/Animal", DoNotGenerateAcw=true)]
	public partial class Animal : global::Java.Lang.Object {
...

that will fail to build for obvious reasons.

Fix is to validate that classes and interfaces belong to a package/namespace. In case namespace does not exist, we should generate class or interface without any namespace.

Tests were added to validate scenario

tracking issue: #519