Make "mark unread" close current message

- XLFragmentManager.selectMailbox() used to hide the message view
  when it's visible, but not anymore.  Made goBackToMailbox() itself
  hide it instead.

- Also made onMarkAsUnread() not call the unread callback, which was
  already done in onMarkMessageAsRead().

Bug 3214538

Change-Id: I52fb8bb745cdaa517624a9aeec074122f10aac4a

src/com/android/email/activity/MessageListXLFragmentManager.java

@@ -360,7 +360,7 @@ class MessageListXLFragmentManager {
      */
     public void goBackToMailbox() {
         if (isMessageSelected()) {
-            selectMailbox(getMailboxId(), false);
+            mThreePane.showLeftPane(); // Show mailbox list
         }
     }
 

src/com/android/email/activity/MessageViewFragment.java

@@ -408,7 +408,6 @@ public class MessageViewFragment extends MessageViewFragmentBase {
 
     private void onMarkAsUnread() {
         onMarkMessageAsRead(false);
-        mCallback.onMessageSetUnread();
     }
 
     /**