Fix NPE crash when start pending predictive back animation.

BackNavigation will start pending predictive back animation after
previous transition finish. While ensuer the visibility of the windows
in previous transition, check whether the surface still exist before
prepareSurfaces.

Flag: EXEMPT bugfix
Bug: 374814683
Test: presubmit
Change-Id: I49faff57249f7b0ce255ea7c8b9f29fec1b97a36