Correct dissapearing SurfaceView during exit animations.

Correct dissapearing SurfaceView during exit animations.

We were missing a call to detachChildren in the case
that the visibility change originates from AM (rather than relayout).
Thus we were racing relayout with the client to detach fast enough.
Detach before notifying the client of window visibility changes.

Test: Slow down animation speed. Open close SV app (say Chrome). No flashes.
Change-Id: Ibf027c2ca295f25765ba95b36c1346a715f3bc6e