Enable auto backup

[davecraig]

Currently we have allowBackup disabled in AndroidManifest.xml because we only had the tile database which is really used as a cache. However, we do want the markers and routes database to be backed up, so we should read https://developer.android.com/identity/data/autobackup and figure out how to correctly manage our caches and database backups so that we can backup the things that are important to us.
We should then check that the Clear storage and Clear cache buttons in the app info/storage and cache tab do what we want.

(Note that there are 'a few' tile caches to consider:

• HTTP cache of tiles from soundscape-backend server
• Realm database of tiles from soundscape-backend server
• Cache of UI map vector tiles from protomap server within maplibre

I think that's all of them!)