[Feature] Support Android built-in backup mechanism (SeedVault)

[jluttine]

Is your feature request related to a problem? Please describe.

It'd be nice to be able to automatically take backups (and then automatically sync those to a remote device). It's cumbersome to do the backups manually because one wouldn't remember/bother to do those frequently.

Describe the solution you'd like

At least recent LineageOS has built-in backup mechanism with SeedVault - I don't know about other Androids. Currently, Photok doesn't support it, but if it did, then Photok data could be backed up together with all other apps. The photos/videos being encrypted of course. This way, Photok wouldn't need to implement its own automatic backup/syncing solutions, but just support the existing SeedVault.

This could be opt-in because some people might not want this to happen by default.

Describe alternatives you've considered

Photok-specific implementation would be an alternative but I think it's better to just support the already feature-rich existing solutions like SeedVault.

Additional context