Fix bug 5135608 - Let AbsListView take focus while detached

While it is valid to request focus while detached from a window,
AbsListView did not handle it gracefully if it had an adapter whose
data changed while the view was detached, since DataSetObservers are
unregistered during this time. Assume that the data could have changed
if we gain focus while detached.

Change-Id: Id135693ffcc48ff5a9d530d335ab449180624c43