Fix flicker with "trampoline" activities

WC.mSurfaceControl starts hidden, but gets immediately shown
in onParentSet. Reflect that in mLastSurfaceShowing in
AppWindowToken such that it gets hidden immediately again while
preparing the first frame.

Test: Go to settings -> Add account, observe no flicker in the
status bar
Test: go/wm-smoke

Change-Id: I6493e75fcac31784662ebcb0e24a4e5c698ed337
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index b24a8ca..44d7948 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -211,7 +211,7 @@
     private int mTransitFlags;
 
     /** Whether our surface was set to be showing in the last call to {@link #prepareSurfaces} */
-    private boolean mLastSurfaceShowing;
+    private boolean mLastSurfaceShowing = true;
 
     private AppWindowThumbnail mThumbnail;