[rilmodem] Allow subscription of stk notifications only once

Allow subscription of stk notifications only when simkit agent registers
for first time. This implementation fixes double subscription problems
which may happen if simkit agent has exited and restarts subscribing to
stk commands again.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
Merge pull request #172 from ptakalok/subscribe

[rilmodem] Allow subscription of stk notifications only once