Make NotificationResponseTransformer respect atNotification.isEncrypted == false

[gkc]

Describe the bug

If a client receives a notification where isEncrypted is explicitly false, it nonetheless continues to treat it as though isEncrypted were true

Steps to reproduce

Came across this bug when I was running an sshnpd with legacy atKeys at the same time as I was doing enroll requests; the enroll request notifications explicitly have isEncrypted == false (which is correct) but NotificationResponseTransformer nonetheless tries to decrypt the data and of course fails

Expected behavior

explicit isEncrypted == false should be respected

Additional context

See also atsign-foundation/at_server#1944

[gkc]

Will work on this once this at_server PR has been merged and rolled out to prod

[gkc]

Will complete in this sprint