if key changed by the counter party, then the "Key has been changed" should be displayed to the user

Please search to check for an existing issue (including closed issues, for which the fix may not have yet been released) before opening a new issue: https://github.com/k9mail/k-9/issues?q=is%3Aissue

Expected behavior

If counter party has changed his/her gpg key, this should somehow be shown to the user.

1. if Autocrypt mode is ON then
   the email message display should contain something like "Key has been changed" and K9 should put the new key into Open keychain automatically

2. if Autocrypt mode is OFF then
   the email message display should contain something like "Key has been changed" and K9 should propose to add the update the key for this counterparty in Open Keychain manually

Actual behavior

Tell us what happens instead
In both cases (Autocrypt ON and OFF) K9 does not display any warnings regarding the change of the key and the new key is being added into the Open Keychain

Steps to reproduce

1. Connect delta chat to the same IMAP account without transitting autocrypt auto setup, so that 2 isntances of deltachat have two different private/public key pairs for same email
2. send email from #1 instance of delta chat to the test account in K9
3. send email from #2 instance of delta chat to the test account in K9
4. Check Open Keychain -- now it has two different keys for the same email and the last one is active for K9 (marked with K9 icon)

Environment

K-9 Mail version:
5.717

Android version:
9

Account type (IMAP, POP3, WebDAV/Exchange):
IMAP

Please take some time to retrieve logs and attach them here:

Start by tracing the email message display and the Autocrypt/Open Keychain handling described in the issue. Reproduce the two-key scenario with Autocrypt enabled and disabled, then verify that a key change is shown to the user and that the corresponding automatic or manual update flow is offered.