Take the /bin/wifi lock in Waveguide scans.

This should reduce how often conman and waveguide race each other at
system boot time.

Also, fix locking:

* Only take the lock once per invocation of /bin/wifi (!!!)
* Try to release the lock when we're done. :)

Change-Id: Ib86e410f3eb23341ab97f92b4619fa099548dee2
5 files changed