Let RemoteViews nest children, allow ViewFlipper.

This change allows applications to nest children RemoteViews
inside an existing set of RemoteViews.  These nested views
are inflated and treated as addView() calls.

This change also allows ViewFlipper through RemoteViews, and
adds logic surpress flipping when the parent window is
detached or behind the lockscreen.

Fixes http://b/2239905