Make DRM libraries optional

Change-Id: I0b591b75a035bc6d8049c85b595966845fedaa15
diff --git a/drm/common/Android.mk b/drm/common/Android.mk
index 249fe8e..808b2c2 100644
--- a/drm/common/Android.mk
+++ b/drm/common/Android.mk
@@ -38,4 +38,6 @@
 
 LOCAL_MODULE:= libdrmframeworkcommon
 
+LOCAL_MODULE_TAGS := optional
+
 include $(BUILD_STATIC_LIBRARY)