Disable proguard for PluginDummyLib

Proguard fails for jars that contain no class files, disable it.

Test: m -j ANDROID_COMPILE_WITH_JACK=false PluginDummyLib
Change-Id: I0001eb3de7b7cea2fa9f91de0c1e59569d41ae8a
diff --git a/packages/SystemUI/plugin/Android.mk b/packages/SystemUI/plugin/Android.mk
index 05ee6b2..e22dddb 100644
--- a/packages/SystemUI/plugin/Android.mk
+++ b/packages/SystemUI/plugin/Android.mk
@@ -36,4 +36,6 @@
 
 LOCAL_JAVA_LIBRARIES := SystemUIPluginLib
 
+LOCAL_PROGUARD_ENABLED := disabled
+
 include $(BUILD_PACKAGE)