Optimize for OfflineGeometryRegionDefinition

[tobrun]

The upstream Mapbox Maps for Android SDK will introduce OfflineGeometryRegionDefinition from mapbox/mapbox-gl-native#11447 in v6.5.0. The codebase for the offline plugin needs to be hardened for occurences of that instance + provide integration for it as well.

Note that in 7.0.0 the interface for OfflineRegionDefinition will change, see mapbox/mapbox-gl-native#12685.

[LukasPaczos]

Blocked by mapbox/mapbox-gl-native#13687.