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)