emacs: tree: use message pane decryption state for replies

This makes replying to a message in tree view, use the decrypted state
from the message pane if it is open. Previously it just used the
global decryption state from notmuch-crypto-process-mime.

In particular if notmuch-crypto-process-mime is nil, and the user
views the messages (which doesn't decrypt), toggles decryption in the
message pane, and then replies, the reply will be decrypted.