plugin-api: deviced: Add display state set attribute

 - DEVICED_DISPLAY_ATTR_TUPLE2_SET_CURRENT_STATE
  : Attribute for setting display state. It requires two parameter,
    each type of enum deviced_display_state and enum deviced_event.
    The first one designates state to be changed and the latter one
    specifies reason why the state change has been triggered.

When it comes to the state setter, it is necessary to reset screen
timeout of the changed state. Therefore the below values have been
added.
 - DEVICED_EVENT_DISPLAY_SCREEN_TIMEOUT
  : Event for screen timeout expiration

 - DEVICED_DISPLAY_SCREEN_TIMEOUT_INFINITE
  : Magic number for setting infinite screen timeout

Change-Id: I616a5ecd009dfb8c5aa4710ed68f2b09f5fe2981
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>