Initial changes to add callback on task stack changes. (Bug 17672056, Bug 18291345)

Add a listener to listen for changes in the Task stacks to preload thumbnails from the
system.  In addition, reduce the amount of synchronous work done in activity creation
and first measure/layout passes.

Change-Id: I8bd9155d7a05e89c190a20429acff69a17808208
diff --git a/Android.mk b/Android.mk
index ce72a85..ab66bec 100644
--- a/Android.mk
+++ b/Android.mk
@@ -69,6 +69,7 @@
 	core/java/android/app/IActivityPendingResult.aidl \
 	core/java/android/app/IAlarmManager.aidl \
 	core/java/android/app/IAppTask.aidl \
+	core/java/android/app/ITaskStackListener.aidl \
 	core/java/android/app/IBackupAgent.aidl \
 	core/java/android/app/IInstrumentationWatcher.aidl \
 	core/java/android/app/INotificationManager.aidl \