base

Destroy saved surfaces if one of the last visible windows gets removed

Also, if by the time the app is closing, a window is still invisible
in layout (or is already removed), mark the window as mAnimatingExit,
so that the surface is destroyed (or saved again). If it's marked
for removal, the window gets removed as well.

bug: 28913302
Change-Id: Ifa3dc0742f9c8c09d741fd64dcdc01b49075628c