[msbuild] Write library project import Jar cache entries correctly (#843)

Author: garuma

src/Xamarin.Android.Build.Tasks/Tasks/ResolveLibraryProjectImports.cs

@@ -105,7 +105,8 @@ public override bool Execute ()
 				var document = new XDocument (
 					new XDeclaration ("1.0", "UTF-8", null),
 					new XElement ("Paths",
-						new XElement ("Jars", string.Join (";", Jars)),
+						new XElement ("Jars",
+							Jars.Select(e => new XElement ("Jar", e))),
 						new XElement ("ResolvedResourceDirectories",
 							ResolvedResourceDirectories.Select(e => new XElement ("ResolvedResourceDirectory", e))),
 						new XElement ("ResolvedAssetDirectories",