Encrypt SMS Database

I'm not sure if this is the way Deku was intended to work, but if it is - consider this a feature request.

**The SMS are not by default encrypted on my device.** By being saved in the standard database that means they're available to all other apps on the device. They should be encrypted by default (like with Silence). I don't want my messages to be accessible to Google or other apps to read on my device without my permission, so they can't be in that open database! Making them a free for all like that really defeats the purpose of using an encrypted SMS app. I'd like to switch from Silence, since its dead, but need the basic SMS & privacy features available there first.

Yes, I realize I can enable encryption for an individual conversations, but that's not what I'm after. That offers the E2E SMS encryption between two Deku users, and what I'm suggesting is messages be encrypted on the device regardless of whether the conversation is E2E (again like Silence).

No files, tests, or entry points are named. Start by tracing how SMS are stored in the standard database and how the existing per-conversation E2E encryption is enabled; done means messages are encrypted at rest by default on the device, independently of E2E conversation encryption.

Written by the indexing model from the issue text.