Incorrect handling of start-of-file noise

[sco1]

An attempt was made in b20f785 to mitigate noise at the beginning of the file causing an incorrect flight start by just trimming some data from the beginning of the file. Unfortunately, this didn't quite fix the problem, at least with the current default:

While the assumption that the FlySight will be on the ground when it's turned on is correct, we need a better way to discard segments that start very near to the beginning of the file. This may be better served with a longer trim default as a quick fix? It's also worth looking at the satellites in view to see if this is just from a lack of accuracy on startup.

I also think this threshold would be better in the flight segment classifier rather than just trimming off the beginning of the data file.