Weird format on OpenPGP encrypt message subjects.

Describe the bug
I have multiple email accounts in the K-9 and only one account has this problem. The problem is that after enable the "Encrypt message subjects", the mails sent with 'sign and encrypt' has a weird format that neither Thunderbird nor Interlink w/ Enigmail can decrypt the encrypted subject. Only pEp can recognize this weird format.

I manually decrypt the weird format between the -----BEGIN PGP MESSAGE----- and -----END PGP MESSAGE----- as below:

Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain;
 charset=utf-8; protected-headers="v1"
Subject: =?UTF-8?B?bW9uZXkg8J+kkQ==?=

happy birthday

So, the problem is this format doesn't provide a independent MIME block for the protected-headers="v1".

I reviewed my other accounts, they all have the mail format with independent MIME block for the protected-headers="v1". So the worked format is that the protected-headers="v1" opens a new nest of MIME block (multipart/mixed) and include the actual mail body in its sub-block.

My conclusion (a guess) is that when you compose a new mail with simple text with the configurations "automatic message format" for the account, then you will get this weird format when K-9 decides to compose the message as plain text.

To Reproduce
Steps to reproduce the behavior:

1. Configure account to enable "Encrypt message subjects"
2. Configure account to "Automatic" "Message format"
3. Compose a new mail with simple text.
4. Enable the encrypt (i.e. sign + encrypt)
5. Send the mail.
6. Receive the mail and read it by Thunderbird and Enigmail powered mail clients.
7. You can't see actual mail subject just "Encrypted Message"

Expected behavior
Users of Thunderbird and Enigmail powered mail clients should be able to get the actual mail subject.

Screenshots
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Environment (please complete the following information):

• K-9 Mail version: 5.806
• Android version: 10
• Device: NOKIA 5.1 Plus
• Account type: IMAP

Additional context
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Logs
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Start by reproducing the issue with "Encrypt message subjects" enabled and the account's message format set to "Automatic". Inspect the MIME structure produced for a plain-text sign-and-encrypt message and compare it with the independent protected-headers MIME block described in the report. Done means Thunderbird and Enigmail can display the actual subject instead of "Encrypted Message".