Make DRM libraries optional

Change-Id: I0b591b75a035bc6d8049c85b595966845fedaa15
diff --git a/drm/libdrmframework/Android.mk b/drm/libdrmframework/Android.mk
index 765adba..99133ba 100644
--- a/drm/libdrmframework/Android.mk
+++ b/drm/libdrmframework/Android.mk
@@ -42,6 +42,8 @@
 
 LOCAL_PRELINK_MODULE := false
 
+LOCAL_MODULE_TAGS := optional
+
 include $(BUILD_SHARED_LIBRARY)
 
 include $(call all-makefiles-under,$(LOCAL_PATH))