Fixes for restoring more than one child surfaces

App may have more than one windows and subwindows. Remember which ones
are visible at the last setAppVisibility time, and restore only those
that was visible then. If the app itself requested to hide a window
before that, we don't want to use the window for early animation.

Also move mAnimatingWithSavedSurface into WindowState as it needs to
be tracked per window.

bug: 27455025
bug: 28296945

Change-Id: I3ed1879634fa7709de699d4518d8fcfc91a85554