Finish migrating KeyAgreementActivity to ViewModel
The code has been refactored substantially (though almost all functionality should have stayed the same), so a faster way to review the code might be to check out the nearby package in Android studio and otherwise look at the diffs of the individual commits.
Test instructions:
- Add two contacts with the nearby option
- try all sorts of variations and complications when doing it:
- revoke permissions in system settings for briar
- rotate screen in odd places through the process (especially before pressing the Next button)
- disable wifi and/or bluetooth of the phone and/or in briar itself (before starting to add contacts)
Closes #1872 (closed)