Create end of animation callback for Activity

Activities cannot draw while their entering animations are active.
This change introduces a callback, onEnterAnimationComplete() so
that activities can know when their draws will be effective.

Fixes bug 13658460.

Change-Id: Ic48540cd4c7e37538f10cb2dc0852aa3f55d11e1
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index 312689b..3fcd067 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -110,6 +110,7 @@
     boolean mDeferRemoval;
 
     boolean mLaunchTaskBehind;
+    boolean mEnteringAnimation;
 
     AppWindowToken(WindowManagerService _service, IApplicationToken _token,
             boolean _voiceInteraction) {