Bluetooth: Avoid cleanup the scan manager if already cleaned up

- Avoid cleanup the scan manager if already cleaned up, scan
  manager can be cleared on receiving the stop request or
  when the service is destroyed.

- There is a case, where destroy and stop called one after
  the other, it will try to cleanup twice, that leads to
  crash.

CRs-Fixed: 797597

Change-Id: I0adc5cb683bbf47f12598e6c54eae7df87cdbbc4