v4.3.0 of geolocator_platform_interface

A common platform interface for the geolocator plugin. Changelog excerpt: - Adds `hasAccuracy`, `hasAltitude`, `hasAltitudeAccuracy`, `hasHeading`, `hasHeadingAccuracy`, `hasSpeed`and `hasSpeedAccuracy`to `Position`, so a value the platform could not measure is distinguishable from the same value measured as `0.0`. The Android platform channel already omits a key when its…