Enable OPGeofence by default

[shankari]

We've been testing the OPGeofence for a while, and it seems

• pretty stable, and
• not leading to too much drain

It is also going to become significantly more important if/when we remove the continuously-on foreground service, since the geofence transition is likely to be even more delayed if we are not in the foreground and are subject to more background restrictions.

https://developer.android.com/training/location/geofencing

Alerts can be late. The geofence service doesn't continuously query for
location, so expect some latency when receiving alerts. Usually the latency is less than 2 minutes, even less when the device has been moving. If Background Location Limits are in effect, the latency is about 2-3 minutes on average. If the device has been stationary for a significant period of time, the latency may increase (up to 6 minutes).

Note: On Android 8.0 (API level 26) and higher, if an app is running in the
background while monitoring a geofence, then the device responds to
geofencing events every couple of minutes.

So let's turn OPGeofence on all the time and hope to get quicker transitions than we would with the classic geofence

• Add a hack that would delete any previous configs so that we don't have zombie configs sitting around confusing things. This can be removed in the next release.