Fix ViewAnimator.getDisplayedChild() bug

Adding a view above the currently displayed child in a ViewAnimator
will cause the displayed child index to become stale, because it is
not incremented to reflect the new position of the currently displayed
view. This fix adds logic to the addView() override to increment the
index correctly.

Change-Id: I9c756ce51a8e8dcbeef8f0e4fbb3704482c21e9d