Incorrect behavior of View clear focus v2.0.

The framework tries to have a focused view all the time. For
that purpose when a view's focus is cleared the focus is given
to the first focusable found from the top. The implementation
of this behavior was causing the following issues:

1. If the fist focusable View tries to clear its focus it
   was getting focus but the onFocusChange callbacks were not
   properly invoked. Specifically, the onFocusChange for
   gaining focus was called first and then the same
   callback for clearing focus. Note that the callback
   for clearing focus is called when the View is already
   focused.

2. If not the first focusable View tries to clear focus,
   the focus is given to another one but the callback
   for getting focus was called before the one for clearing,
   so client code may be mislead that there is more than
   one focused view at a time.

3. (Nit) The implementaion of clearFocus and unFocus in ViewGroup
   was calling the super implementaion when there is a
   focused child. Since there could be only one focused View,
   having a focused child means that the group is not focused
   and the call to the super implementation is not needed.

4. Added unit tests that verify the correct behavior, i.e.
   the focus of the first focused view cannot be cleared
   which means that no focus change callbacks are invoked.
   The callbacks should be called in expected order.
   Now the view focus clear precedes the view focus gain
   callback. However, in between is invoked the global
   focus change callback with the correct values. We may
   want to call that one after the View callbacks. If
   needed we can revisit this.

Change-Id: I8cfb141c948141703093cf6fa2037be60861cee0