Fix NPE when destroyActivity occurs after detach.

 *** FATAL EXCEPTION IN SYSTEM PROCESS: ActivityManager
 java.lang.NullPointerException
 	at com.android.server.am.ActivityStack.removeTask(ActivityStack.java:3627)
 	at com.android.server.am.ActivityStack.removeActivityFromHistoryLocked(ActivityStack.java:2693)
 	at com.android.server.am.ActivityStack.activityDestroyedLocked(ActivityStack.java:2878)
 	at com.android.server.am.ActivityStack$ActivityStackHandler.handleMessage(ActivityStack.java:296)
 	at android.os.Handler.dispatchMessage(Handler.java:102)
 	at android.os.Looper.loop(Looper.java:136)
 	at com.android.server.am.ActivityManagerService$AThread.run(ActivityManagerService.java:1868)

Change-Id: Ifaf0fe6c62d1f63897d57dc96b4d86d8111b45db