NSLocationWhenInUseUsageDescription in Info.plist is always empty and gets reset to empty whenever the app is built. This improves the process of adding the plugin so that the value is always set

When the geolocation plugin is added using the command
cordova plugin add cordova-plugin-geolocation
no value is added in the plugin.xml file for the NSLocationWhenInUseUsageDescription even though it is in there it is empty. Updating this Key directly in the plist and it always gets reset to empty. Updating it in the plugin.xml means removing and adding or updating the plugins might overwrite the value.
This fix simply adds it to the config.xml so that it can be specified on installation using --variable or modified later by changing the value in the config.xml, removing the plugin using cordova plugin rm and then running cordova prepare which re-adds the plugin with the updated value of the key