Add support for GST sentences

[GeoSpark]

Currently the code records the horizontal accuracy as a multiple of HDOP which has little correlation with the 1-sigma error in metres that the Android Location class expects. Some GNSS receivers can output GST sentences that encode both the horizontal and vertical errors. See https://gpsd.gitlab.io/gpsd/NMEA.html#_gst_gps_pseudorange_noise_statistics