Part of ENG-3100

This diff adds support for storing encrypted media messages in client db.

I didn't modify the ClientDBMediaInfo type - it still contains only photo | video. Deduction if it is encrypted is based on existence of the encryption_key property. This approach works well, but I don't have strong opinion on it, I can refactor if requested.

Depends on D7169

Added unit tests. Also modified keyserver to compose encrypted media messaes and checked if they're properly saved and restored on native.