Prevent possible leak in AbsListViews.

This change unregisters a list's data observer from the adapter when the list
is detached from the window. The default data observer impl. is a non-static
inner class with a synthesized pointer to the list. Thus if an app keeps
the adapter around the list is also kept around. This can be a problem with
dialogs in particular.

Change-Id: Ia95a995d3f54b9caf1bbcd2c7239a0318511b08c