conman:  Experiment to prevent joining the WLAN on 2.4 GHz.

If WifiNo2GClient is enabled, conman will not start a 2.4 GHz client.

Being able to test this required a small refactor of both actual and
test code.

BUG=30199783

Change-Id: I1c351e341bafa360727ecbca1bf70b629387a890
2 files changed