fix up! Remove thread for preview callback when stop preview

fix up! Remove thread for preview callback when stop preview

- The deadlock is occurred with below sequence.
  1. camera_start_preview() -> create thread for preview callback
  2. camera_start_capture()
  3. got capture_completed_cb()
  4. camera_start_preview() -> create new thread for preview callback and the old one is remained.
  5. camera_stop_preview() -> send signal to remove thread for preview callback, but 1 thread is remained.

[Version] 0.4.68
[Issue Type] Bug fix

Change-Id: Id137ef203b1e0c85f597c05ae818ecb2a1b6c679
Signed-off-by: Jeongmo Yang <jm80.yang@samsung.com>