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

services/core/java/com/android/server/wm/WindowState.java

@@ -1528,6 +1528,13 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
             return changed;
         }
 
+        // Next up we will notify the client that it's visibility has changed.
+        // We need to prevent it from destroying child surfaces until
+        // the animation has finished.
+        if (!visible && isVisibleNow()) {
+            mWinAnimator.detachChildren();
+        }
+
         if (visible != isVisibleNow()) {
             if (!runningAppAnimation) {
                 final AccessibilityController accessibilityController =