Account screen: Provide option to clear local database

[ryanw-mobile]

Child ticket of #217

The UX of downloading all half-hourly usage and rates has to be studied - this also affects the usage view.
For now, since we have RoomDB implemented and started storing local data, we should first allow clearing the DB.

This should be done by:

1. Without clearing credentials, just clear the local DB storage, or
2. Clear the DB when clearing credentials and return to demo mode,

For accounts having multiple meters, we retain the cache data (as they are linked to meter serial), so that switching between meters won't force everything to be downloaded. -- and that is why we need a separate button to clear the local DB.