[Xamarin.ProjectTools] Fix path to xabuild for monodroi. (#948)

The Builder was not checking for the xabuild script in the
`out` directory for the monodroid tree.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/Builder.cs

@@ -50,6 +50,9 @@ namespace Xamarin.ProjectTools
 					#else
 					var xabuild = Path.GetFullPath (Path.Combine (Root, "..", "Release", "bin", "xabuild"));
 					#endif
+					if (File.Exists (xabuild))
+						return xabuild;
+					xabuild = Path.GetFullPath (Path.Combine (Root, "..", "..", "..", "..", "..", "..", "..", "out", "bin", "xabuild"));
 					if (File.Exists (xabuild))
 						return xabuild;
 					return Path.GetFullPath (Path.Combine (Root, "..", "..", "tools", "scripts", "xabuild"));