Issue title: Change location accuracy for Google Analytics threshold
 Issue body: @cagryInside I was looking at some of the location data from a few Android phones, and I'm thinking it probably makes sense to tighten up the horizontal accuracy threshold prior to a production release.

Currently, [we're using 100m](https://github.com/OneBusAway/onebusaway-android/blob/master/onebusaway-android/src/main/java/com/joulespersecond/oba/ObaAnalytics.java#L43) as the cutoff threshold - meaning that we report analytics for distance to stop if the estimated error radius of the location point is less than 100m, and throw it away if its >= 100m.

I'm thinking it may be better to change this to 50m, so we have a tigher bound/accuracy on the analyitics data.  Looking at a few devices, most of them report accuracy < 50m when its reasonable.  If its more than 50m, it starts getting questionable, especially for our [smallest radii](https://github.com/OneBusAway/onebusaway-android/blob/master/onebusaway-android/src/main/java/com/joulespersecond/oba/ObaAnalytics.java#L51), which is defined at 50m separation.

Your thoughts?  Any data from your devices?

Good apps to test with are [GPS Benchmark](https://play.google.com/store/apps/details?id=com.gpsbenchmark.android) and [GPSTest](https://play.google.com/store/apps/details?id=com.android.gpstest) (yes, they are both my apps ;) ).
 Comments: 
 Comment 0: @barbeau In the high-end device (nexus 6) I usually measure the horizontal accuracy <30m. But, I think we should also measure some results with older devices to see the variation.

 Comment 1: I was looking at data I collected from a number of Android 2.1 and 2.2
devices a while back, and it seems consistently <50m are good locations.
On Feb 27, 2015 12:14 PM, "Cagri Cetin" <robin24@example.net> wrote:

> @barbeau <https://github.com/barbeau> In the high-end device (nexus 6) I
> usually measure the horizontal accuracy <30m. But, I think we should also
> measure some results with older devices to see the variation.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/OneBusAway/onebusaway-android/issues/232#issuecomment-76431617>
> .
>

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