Fix a monkey bug with AbsListView data set observers

Several conditions can cause an AbsListView's data set observer to be
removed and nulled out. If for some reason the view receives duplicate
onDetachedFromWindow events this could cause AbsListView to attempt to
unregister a null observer. Skip this unregister process if this
happens.

Bug 7088152

Change-Id: Ib0c630d1ee598640512023e4ef158f01e3ed474d