libsnapshot::RemoveAllUpdateState log stack

Bug: 148818798
Test: apply OTA but cancel it in the middle, then
      delete /data/misc/update_engine/prefs/*, then
      reapply it again (to trigger CancelUpdate()).
      Stack is dumped.

Change-Id: I2dfb6b7f15b9d5854d0b81ed8ad260538d21c922