Migrate AppWindowToken lists into DisplayContent.

In preparation for converting ActivityManager control to a task-based
interface the AppWindowTokens are being stored per-display.

Change-Id: Ie5e355219554523f5e56eaef138d382975cf1682
diff --git a/services/java/com/android/server/wm/AppWindowToken.java b/services/java/com/android/server/wm/AppWindowToken.java
index 0ada604..8fd5209 100644
--- a/services/java/com/android/server/wm/AppWindowToken.java
+++ b/services/java/com/android/server/wm/AppWindowToken.java
@@ -30,6 +30,10 @@
 import android.view.WindowManager;
 
 import java.io.PrintWriter;
+import java.util.ArrayList;
+
+class AppTokenList extends ArrayList<AppWindowToken> {
+}
 
 /**
  * Version of WindowToken that is specifically for a particular application (or