Allowing association between a view and its label for accessibility.

1. For accessibility purposes it is important to be able to associate
   a view with content with a view that labels it. For example, if
   an accessibility service knows that a TextView is associated with
   an EditText, it can provide much richer feedback.

   This change adds APIs for setting a view to be the label for another
   one and setting the label for a view, i.e. the reverse association.

bug:5016937

Change-Id: I7b837265c5ed9302e3ce352396dc6e88413038b5