harden a bit NetworkHelper methods and calls

A crash was detected in some case. For example, at boot time, when we
have previously enabled LAN bypass, it may happen the BootReceiver
crashes because orWall is unable to get the Wifi IP address — in case
the network wasn't already up, for example.
Allow LAN bypass to be triggered from NetworkReceiver

In previous patch, a bug was fixed. This new patch allow orWall to
manage in a better way the LAN bypass (though it's still not perfect).

Next step would be: in case we don't get any IP on the Wifi, we should
ensure no deprecated rule stay in IPTables. This means we should be able
to register the LAN subnet in order to take it back.
This could be done un the sharedPreferences.
added comment
removed useless call, added TODO
design/placement