Stop restoring tasks added before recent is loaded

Previously we can have duplicated tasks sharing the same taskId in
RecentTasks due to race condition.

1. A task is created before RecentTasks#loadUserRecentsLocked
   (e.g. through adb)
2. RecentTasks#notifyTaskPersisterLocked eventually writes the task file
   to storage (e.g. XX_task.xml)
3. RecentTasks#loadUserRecentsLocked tries to recover XX_task.xml while
   the task has already been added to RecentTasks.

To fix the issue, the CL stops restoring tasks added before recent is
loaded.

Bug: 36796576
Test: Build and boot Android, check the recent is correctly loaded
Change-Id: Ib57977f2a0a63f7bf7db4d3fd70bdcc359e76f7d