K9 uses default account when clicking mailto link in secondary account.

Describe the bug
When multiple accounts are setup and one is selected as the default sending account, mailto links opened from emails in the other accounts will still use the default account as sender.

To Reproduce
Steps to reproduce the behavior:

1. Setup multiple accounts.
2. Set one of the accounts as the default account for sending emails in the account settings.
3. Open an email in one of the "non-default" accounts.
4. Click on a mailto link (e.g., unsubscribe in a newsletter or such).
5. The new email will have the default acocunt selected as sender instead of the current account.

Expected behavior
I cannot think of any situation in which I would want to open an email-link from a mail in account B but want send a mail from account A instead. While the sender can of course be changed during drafting the new email, I think the more natural behaviour would be to answer the mailto link from the account that received the message containing said link.

Environment (please complete the following information):

• K-9 Mail version: 5.8.0.6
• Android version: 10
• Device: Google Pixel A3

Reproduce the behavior with multiple accounts on Android 10 using the listed steps, then trace the mailto-link handling from an email opened in a non-default account. Done means a mailto draft opened from that message selects its account as sender rather than the default account, with the relevant behavior covered by tests if the existing project structure provides them.