Make DRM libraries optional

Change-Id: I0b591b75a035bc6d8049c85b595966845fedaa15
diff --git a/drm/jni/Android.mk b/drm/jni/Android.mk
index 95ce75a..b65e4da 100644
--- a/drm/jni/Android.mk
+++ b/drm/jni/Android.mk
@@ -44,5 +44,7 @@
 
 LOCAL_PRELINK_MODULE := false
 
+LOCAL_MODULE_TAGS := optional
+
 include $(BUILD_SHARED_LIBRARY)