Add ability to cancel task window transitions.

Bug: 25392381

Change-Id: I45f48edc21c058df0e4c22ceaf7e9aef5899a29c
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index ad44196..d4d99af 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -320,6 +320,15 @@
         }
     }
 
+    /**
+     * Cancels any running app transitions associated with the task.
+     */
+    void cancelTaskWindowTransition() {
+        for (int activityNdx = mAppTokens.size() - 1; activityNdx >= 0; --activityNdx) {
+            mAppTokens.get(activityNdx).mAppAnimator.clearAnimation();
+        }
+    }
+
     boolean showForAllUsers() {
         final int tokensCount = mAppTokens.size();
         return (tokensCount != 0) && mAppTokens.get(tokensCount - 1).showForAllUsers;