Export identities that have no description

Identities must have an email address but all of the other fields are
optional, so email address should be used to determine identity keys.

Fixes #6932

app/core/src/main/java/com/fsck/k9/preferences/SettingsExporter.kt

@@ -206,7 +206,7 @@ class SettingsExporter(
             if (hasThirdPart) {
                 val secondPart = keyPart.substring(0, indexOfLastDot)
                 val thirdPart = keyPart.substring(indexOfLastDot + 1)
-                if (secondPart == IDENTITY_DESCRIPTION_KEY) {
+                if (secondPart == IDENTITY_EMAIL_KEY) {
                     // This is an identity key. Save identity index for later...
                     thirdPart.toIntOrNull()?.let {
                         identities.add(it)