Return the order key in the conversation cursor.

For Email the order key is the message timestamp, Gmail sortMessageId.  The
order key is then used as the sort key for notificatin api, and because it is
stable, notification bridger can avoid bridging duplicate notifications and
reodering the email card bundle on the watch.

Bug 15563260

Change-Id: Ie68f45de91cc42c58238b7cdc0ae828048e22a46

src/com/android/email/provider/EmailProvider.java

@@ -2458,6 +2458,7 @@ public class EmailProvider extends ContentProvider {
                 .add(UIProvider.ConversationColumns.ACCOUNT_URI,
                         uriWithColumn("uiaccount", MessageColumns.ACCOUNT_KEY))
                 .add(UIProvider.ConversationColumns.SENDER_INFO, MessageColumns.FROM_LIST)
+                .add(UIProvider.ConversationColumns.ORDER_KEY, MessageColumns.TIMESTAMP)
                 .build();
         }
         return sMessageListMap;