Add new log for DocsUI

Test: manual
Test: ./statsd_testdrive -p com.google.android.documentsui 110
Bug: 146413601
Change-Id: Id94e67220e80a85383dd0495abafe0c7dfe72a3f

core/proto/android/stats/docsui/docsui_enums.proto

@@ -56,6 +56,7 @@ enum Root {
     ROOT_VIDEOS = 9;
     ROOT_MTP = 10;
     ROOT_THIRD_PARTY_APP = 11;
+    ROOT_DOCUMENTS = 12;
 }
 
 enum ContextScope {