v6 cannot read offline packages from v5
v6 cannot read offline packages from v5

This PR fixes issue https://github.com/mapbox/react-native-mapbox-gl/issues/735. 

v5 and v6 have different ways of storing metadata of offline packs. In v6 we are expecting string JSON while in v5 it was NSDictionary.
bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl