Fix missing column name and make it final.

Change-Id: I371511a293fbe8a858ecb492511b2f5a31583913
diff --git a/src/com/android/mail/providers/UIProvider.java b/src/com/android/mail/providers/UIProvider.java
index 881c661..7f872de 100644
--- a/src/com/android/mail/providers/UIProvider.java
+++ b/src/com/android/mail/providers/UIProvider.java
@@ -504,7 +504,7 @@
          * This string column contains a specially formatted string representing all
          * attachments that we added to a message that is being sent or saved.
          */
-        public static String JOINED_ATTACHMENT_INFOS = "joinedAttachmentInfos";
+        public static final String JOINED_ATTACHMENT_INFOS = "joinedAttachmentInfos";
 
         private MessageColumns() {}
     }