Store current service state from engine library

Store current service state from engine library

- Requirement:
Client library needs to get current service state when preparing is
succeeded.

- Contents:
This patch makes client library store the service state in client
structure when preparing is succeeded. If the client listen the service
state changed event, client has to maintain the current service state
even if the service is out. To do this, each client structure has to
store the current service state.
Through this patch, client structure can maintain the latest service
state at preparing success.

Change-Id: If2ea7b9ebc627363f0b16c51a918d2910a4bcd26
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>