Fixing a memory leak in ViewRootImpl and a focus change callback issue.

1. ViewRootImpl was keeping reference to the old focused view so it can
   call back the global on focus change listener when another view gets
   focus. The stashed reference, however was not cleared which caused a
   memory leak if the last focused view was removed from the view tree.
   In general keeping additional state for the last focus in ViewRootImpl
   is not a good idea since this add complexity due to additional book
   keeping work that is required. The view tree already keeps track of
   where the focus is and it should be the only place that holds this
   data. Since focus does not change that frequently it is fine to look
   up the focus since this operation is O(m) where m is the depth of the
   view tree. This change removes the duplicate book keeping from
   ViewRootImpl and the focus is looked up as needed.

2. ViewRootImpl was calling the global focus change callbacks when focus
   is gained, lost, or transferred to another view. This was done in
   *ChildFocus methods. In the case of a child losing focus, i.e. in
   clearChildFocus, there was a check whether focus searh yields a view
   to take focus and if so it did not call back the global focus listener
   assuming the the found view will take focus (the view tree gives focus
   to the first focusable when a view looses focus). This is not a correct
   assumption since some views override methods called as a result of
   View.requestFocus that determine what the next focused view should
   be. For example, HorizontalScrollView overrides onRequestFocusInDescendants
   and changes the direction of the search. In fact focus search does not
   take into accound ViewGroup descendant focusability. Hence, the view found
   by calling the focus search from the root is not necessarily the one
   that will get focus after calling requestFocus. Actually, it is
   possible that the focus search will find a view but no view will
   take focus. Now the View class is responsible for calling the
   global focus listeners which avoids the above problem. Also this
   saves book keeping in ViewRootImpl.

bug:7962363

Change-Id: Ic95a18b364e997021f3f6bb46943559aac07d95a