Make needsCheckpoint cover whole session

This makes needCheckpoint return true when the device will or is using
checkpointing.

Test: vdc checkpoint startCheckpoint 1
      reboot
      vdc checkpoint needsCheckpoint
      should return 1 before and after data mounts, and 0 once the
      checkpoint has been committed

Change-Id: Ib57f4461d837f41a8110ed318168165a684d913a