[Offline map imagery] Make offline imagery source URLs fully configurable

[gino-m]

We currently only support COGs with low-res world.tif at zoom levels 1-7, and hi-res levels 8-14. These levels and URL subpaths are hard-coded in GoogleMapsFragment. Instead we should use a standard metadata format (STAC?) to specify bounds, zoom levels, and URLs of various tile sets.

[sufyanAbbasi]

See #2115, it's easier to structure the TIFFs by zoom level in Firebase Cloud Storage and query it to determine the available zoom levels.