Fixed AAR processing failure when an AAR without classes.jar is found.

Fixes googlesamples#195

Bug: 132071668
Change-Id: Ic0f076c0b745dcb7e8a7e6bbe36a6a9251dee6f1

Author: Stewart Miles

source/PlayServicesResolver/src/DefaultResolver.cs

@@ -309,6 +309,7 @@ internal bool ProcessAar(string dir, string aarFile, bool antProject,
                         // project or AAR without a classes.jar, the build is aborted.  This
                         // generates an empty classes.jar file to workaround the issue.
                         string emptyClassesDir = Path.Combine(stagingDir, "empty_classes_jar");
+                        Directory.CreateDirectory(emptyClassesDir);
                         if (!ArchiveAar(targetClassesFile, emptyClassesDir)) return false;
                     }
                 }