Don't destroy surfaces of activities launched behind early.

When the launch activity behind feature was introduced in
http://ag/502868, we destroy the surfaces of windows associated with
the activity early in the last app window animation step. The causes
issues with activities that might be using the surface and not
prepared to have the surface destroyed from underneath them. We now
let the surface destruction get handled like any other window so all
the components/processes interested in the surface can be in the
right state when the surface is destroyed.

Bug: 18325222
Change-Id: I45e834a07a199b8a9e364f7392db99c871a43280