Add "%s selected" subtitle to Conversation List batch mode.

While looking how to implement the action bar in the coversation itself, I had a look at how it was done in the conversation list and noticed that the xml there was containing code to show how many items currently are selected - but this code was never used.

Also, this can be done simply by mode.setTitle and mode.setSubtitle, no need for a whole xml. So I removed the xml and implemented the code.

Now, when more than 1 item is selected, the subtitle shows "%s selected" (new string to be translated).
No abbreviated variable names please.  Also, context.getString() supports string format arguments, so you don't have to wrap it in String.format.  Probably best just to do all this inline.