[generator] Prevent BG8A08 warning when using ns-replace (#1136)

Context: 0c90cf5
Context: be2acbc

As part of the Nullable Reference Type work in be2acbc, we added a
BG8A08 warning when the `//*/@path` attribute of a "metadata" element
is not provided.  However, metadata files can also contain
`<ns-replace/>` elements, which do not have a `@path` attribute, and
thus erroneously trigger this warning:

	generated\msbuild-metadata.xml(3,4): warning BG8A08:
	Metadata.xml element '<ns-replace source="com.google.androidx" replacement="Xamarin.AndroidX" />' is missing the 'path' attribute.

Skip running any of the standard "metadata" logic when we hit a
`<ns-replace>` element, as they are handled elsewhere.

Author: jpobst

src/Java.Interop.Tools.Generator/Metadata/FixupXmlDocument.cs

@@ -41,6 +41,10 @@ public void Apply (ApiXmlDocument apiDocument, string apiLevelString, int produc
 				if (!ShouldApply (metaitem, apiDocument))
 					continue;
 
+				// Namespace replacements are handled elsewhere
+				if (metaitem.Name.LocalName == "ns-replace")
+					continue;
+
 				var path = metaitem.XGetAttribute ("path");
 
 				if (path != prev_path)