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/am/ActivityManagerService.java b/services/java/com/android/server/am/ActivityManagerService.java
index e0046ad..55c2e2a 100644
--- a/services/java/com/android/server/am/ActivityManagerService.java
+++ b/services/java/com/android/server/am/ActivityManagerService.java
@@ -194,7 +194,7 @@
     static final boolean DEBUG_POWER_QUICK = DEBUG_POWER || false;
     static final boolean DEBUG_MU = localLOGV || false;
     static final boolean DEBUG_IMMERSIVE = localLOGV || false;
-    static final boolean VALIDATE_TOKENS = false;
+    static final boolean VALIDATE_TOKENS = true;
     static final boolean SHOW_ACTIVITY_START_TIME = true;
     
     // Control over CPU and battery monitoring.