Remove the ListSelectionManager / message body selection (fixes memory leak!)

- When the `viewHolder.messageBody` `TextView` created by a `MessageAdapter` is
  set to selectable, it leaks an `android.widget.Editor` (because that editor
  registers a view observer that never gets unregistered).
  - This memory leak is really quite problematic, as the message adapter is used
    a lot!
- Having the text be selectable is useless anyway, though; there isn't any way
  to select it (because long pressing just opens the context menu anyway).
  - It looks like the ListSelectionManager was meant to track selections across
    multiple messages. However, I'm not sure this feature ever gets used.
- Accordingly, this commit removes the entire feature, thus fixing the memory
  leak (since no `Editor` objects are ever created).
  - It should also reduce memory usage in general, since we aren't attaching an
    `Editor` to every single textview we create.
  - A `TextView` only allocates an `Editor` if you ask it to do certain things,
    like make the text selectable or register custom selection callbacks.