Track added/removed window when focus is null

Improves the blaming of ANRs by determining if an app added windows
after it was launched.

Bug: 38121026
Test: Log inspection
Change-Id: Id86c1981150cf546c92d786ad45d5c2dda5573e0
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index 818df01..bfebca8 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -163,6 +163,8 @@
 
     @VisibleForTesting
     boolean shouldDeferRemoval() {
+        // TODO: This should probably return false if mChildren.isEmpty() regardless if the stack
+        // is animating...
         return hasWindowsAlive() && mStack.isAnimating();
     }