[Mirroring][Add] Add internal api for checking screen mirroring sink state.
-Initial version.
-It returns 'scmirroring_sink_state_e' when it is called.
-It will be robust after about exception case.
[Version] 0.1.224
[Profile] Common
[Issue Type] Add internal api
[Dependency module] screen-mirroring,https://review.tizen.org/gerrit/#/c/136120/
[Test] [M(T) - Boot=(OK), sdb=(OK), Home=(OK), Touch=(OK), Version=tizen-unified_20170620.1]
Change-Id: I422f9b25fb573421141770004fb07b45c6e500e7
Signed-off-by: Hyunsoo <hance.park@samsung.com>