Provide import/export functionality

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Provide import / export functionality

### Motivation

As a user, I want to be able to export my Cryptomator for Android settings and vault configuration in a password-protected way. When I want to import them again, the app should be in the same state as before the export.

### Considered Alternatives

Create plaintext backup without credentials (without cloud connection- and vault passwords).

### Anything else?

This topic isn't that simple as it sounds as we need to have different app versions in mind and handle that accordingly.