[k-9-mail] Flowcrypt + GMail encryption: K-9 Mail requests access to one of your keys...this key is not available

[Caleb Callaway]

I've set up K-9 mail + OpenKeychain
<https://k9mail.github.io/documentation/security/pgpmime_current.html> on
my up-to-date Google Pixel smartphone (Android v9). When I try to open a
PGP encrypted email message in K-9, I get a modal dialog titled "Allow
access to your key?" and a message, "This key is not available. To use it,
you must import it as one of your own!" The PGP key of the *sender* is
listed as the "Requested key". The sender's key is very definitely
available and verified in OpenKeychain. I can decrypt the email in both
Flowcrypt and in Evolution.

The fix had nothing to do with the PGP key of the sender; I had in fact
mistakenly deployed a stripped version of my own key that didn't include
the subkey used to encrypt the email, so verifying the PGP key of the
sender was a "red herring." This looks like a usability bug to me, but I
don't know which app is actually producing the misleading message. Does the
information about the requested key coming from K-9?