without this line, we will always rebuild classes.dex

lib/motion/project/template/android.rb

@@ -345,6 +345,7 @@ EOS
   end
 
   # Compile java files.
+  classes_changed = false
   vendored_jars = App.config.vendored_projects.map { |x| x[:jar] }
   vendored_jars += [File.join(App.config.versioned_datadir, 'rubymotion.jar')]
   classes_dir = File.join(app_build_dir, 'classes')