Add a confidence threshold for activity detection

[barbeau]

Is your feature request related to a problem? Please describe.
Looking at data from a Pixel 5, there seem to clearly be activity transitions that the device is more confident in. Some have a "Google Activity Confidence" of .1, while others have .88 or .9.

Describe the solution you'd like
It would be nice to have another command-line parameter where we could pass in a threshold for activity confidence, where activity transitions would only be exported if they were over the threshold value.

Note that this might create noise in the data if we have an activity with a strong confidence for a ENTER transition and weak confidence for an EXIT transition - we might be looking for another end transition that never comes. So we should probably treat these in pairs - if there is a low ENTER transition too, then throw out the EXIT transition too.