Mark DocumentsUI as a "privileged" app.

This fixes the issues with CTS resulting from the change to intent filtering.
Bug: 28081616

Change-Id: I81f87ff56b5fbb0d526863da44a2fcef29a3aa2c
diff --git a/Android.mk b/Android.mk
index 3197abd..568e200 100644
--- a/Android.mk
+++ b/Android.mk
@@ -2,6 +2,7 @@
 include $(CLEAR_VARS)
 
 LOCAL_MODULE_TAGS := optional
+LOCAL_PRIVILEGED_MODULE := true
 
 LOCAL_SRC_FILES := $(call all-java-files-under, src)