stk: simplify respond_on_exit flag handling

stk: simplify respond_on_exit flag handling

respond_on_exit flag is set by commands which are dispatched to the
agent, so that if the agent exits prematurely, a 'User Terminated
Session' response is sent to the SIM.

There were a couple of corner cases not quite handled correctly:

- During Set Up Call, if the user confirmation phase succeeded and the
  call was dispatched to voicecall atom successfully, and the agent
  exited at this point, then no terminal response would be sent until
  the call succeeded / failed.  Now the agent termination results in an
  'User Terminated Session' response being sent immediately, but the
  call setup proceeding.