PandoraServer: Replace usage of shutdownNow with shutdown

Calling shutdown let us stop accepting new incoming connections
and let the current ones finish.

This is safer for PandoraServer than shutdownNow because we needed
to call shutdownNow just after the completion of Reset as we
wanted to respond to the client before restarting the server.
But this could let the client make another call that the server
could accept before the server shutdown.
This should be prevented with shutdown as after shutdown
return to the caller the server should not accept incoming connections
anymore.

So now we stop accepting incoming connections before responding
to the client instead of after.

Bug: 244615429
Test: atest -v pts-bot --iterations 4
Change-Id: If58bfedc053168fcce2e4c6e1364344ae489e3d5