- 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.
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