[Offline base maps] Add support for web tile layers

Project managers should be able to set up one or more web tile layers in the project by providing a tile source URL. The layers should be toggleable in the Android app under the layers dialog, downloadable via the offline areas UI. Idea is still a WIP. @coreyleamon for UX guidance, @scolsen for eng.

Potential PR split:

- [ ] View (not download) from remote web tile source in main map and offline area selector map. (pick up from from #775).
- [ ] Download for offline use, store in db.
- [ ] Fetch from db first if available, else fetch from remote.
- [ ] Add tile caching

Then https://github.com/google/ground-android/issues/936

Then if we think we can get tiles:

- [ ] Add tile retention policy - alert when expiring, prompt to re-download, force delete of expired tiles.