power: unify power state transition routine

power: unify power state transition routine

 Previously, the iot-headless dedicated poweroff was working on top of
the core poweroff routine. Integrate them into the single poweroff
routine so that all type of poweroff is now take the same subroutine.

 In addition to this, there two internal functions have been added for
deviced to trigger change power state by itself. All power state change
thorugh these functions take the same subroutine.
 - power_request_change_state_strict()
 - power_request_change_state()

 Due to the above changes, the deviced is now able to broadcast all
types of power change state in the unified manner. Therefore, if
someone wants to know the power state change of deviced, just use
power API.

Change-Id: I6573872eb3aa7c50d2d6af575ccce4dc25cab3de
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>