[rilmodem] Fine tuning of start up sequence

Register to listen STK specific unsolicited messages only if UI STK agent has
registered. This because RILD will send and buffer proactive commands
also if not received RIL_REQUEST_REPORT_STK_SERVICE_IS_RUNNING
previously. RILD will buffer the proactive command until receiving
RIL_REQUEST_REPORT_STK_SERVICE_IS_RUNNING or terminal
response to (sent) proactive command.
By this change oFono core/rilmodem will not handle proactive commands
until STK UI agent has been registered to oFono.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>