[rilmodem] Add support for RIL_REQUEST_SCREEN_STATE request

Support for RIL_REQUEST_SCREEN_STATE request will decrease power
consumption as RILD will notify the baseband to suppress certain
notifications (eg. signal strength and changes in LAC/CID, etc.).
Implementation listens to MCE's "display_status_ind" DBus signal for
sending  RIL_REQUEST_SCREEN_STATE.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #83 from ptakalok/master

[rilmodem] Add support for RIL_REQUEST_SCREEN_STATE request