[usb-moded] Add connman support to switch wifi off

Connman might switch wifi back on, which keeps us from activating or keeping
the cellular connection activated. Now we power wifi down if needed, and back
on after USB gets unplugged if it was turned on originally.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
[usb-moded] Do not do post-sync after mode setting error.

There is no point in running the post-sync if the mode setting failed.
So we skip it in that case (also makes sure that the services are not
active in the failure case)

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>