Adds functionality to automatically test new networks

[aanorbel]

Fixes ooni/probe#900

Proposed Changes

• Save last_known_network (ASN)
• Register broadcast receiver (ConnectivityReceiver) to listen to connectivity change and trigger new test.

[hellais]

If I understand this feature correctly, we are adding a new settings which affects the starting of background tests to also be triggered when a network is changed. Is that correct?

If so, do we also consider the options related to background tests when determining if it should run (eg. wifi only, only on power)?
If we do consider these options, what happens if the user is switching from wifi to mobile frequently? Will we be re-triggering the tests very often?

I also wonder if we want to somehow listen to this event while we are running a test too and if we notice a network change during a running test operation, we can interrupt it (this would allow us to detect the changes in network and avoid mislabeling measurements with the wrong network information).