Bluetooth : fix for handling back key during pair dialog display

- Pressing back key when pairing dialog appears infront , device shows "Pairing..."
  if we don't have any bonded device in device list.
- when press back key will perform onCancel() operation.

Change-Id: Iacd654efad455f17a5c3bb09f822d9af5a7aed7d
Signed-off-by: jhtop.kim <jhtop.kim@samsung.com>