CB-8085 Fix geolocation plugin on Windows

https://issues.apache.org/jira/browse/CB-8085

This patch adds keepCallback property to tell proxy preserve callback so that it could be invoked next time.

When running on Windows Phone 8.1: added dummy 1 meter value for geolocator.movementThreshold to prevent the following error (no such problem  when running on Windows 8/8.1)
```
// JavaScript runtime error: Operation aborted
// You must set the MovementThreshold property or the ReportInterval property before adding event handlers.
            
```
http://blogs.windows.com/buildingapps/2012/12/03/geoposition-advanced-tracking-scenarios-for-windows-phone-8/