wifi: Tell wpa_supplicant to keep scanning

It seems by default, wpa_supplicant won't keep a list of BSS around but
only if it has done a scan recently. This means we currently end up with
very bad accuracy since we only send out the ESSID of the currently
connected WiFi (if any).

To fix this, we'll now tell wpa_supplicant to start a scan when the source is
started. We keep doing that continuously if accuracy is street level or higher,
but wait 5 minutues between scans if requested accuracy is lower than that.

Fixes #91.
3 jobs for keep-scanning in 3 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #68966
ubuntu-16.04

00:03:21

passed #68967
ubuntu-17.10

00:01:53

passed #68968
ubuntu-18.04

00:03:02