Allow backup to Seedvault

**Is your feature request related to a problem? Please describe.**
Having to set up apps without a Google app backup used to be a pain. Now there is Seedvault as a backup solution integrated into LineageOS 17.1.

**Describe the solution you'd like**
Element Android should allow itself to be backed up to Seedvault.

**Describe alternatives you've considered**
A manual backup which creates a file in the internal storage.

**Additional context**
https://github.com/vector-im/element-android/issues/2588#issuecomment-752002058

Hmmm. Apparently, it may not be a good idea: https://github.com/seedvault-app/seedvault/wiki/FAQ#why-do-some-apps-not-allow-to-get-backed-up