K9 does not appear to respect system HTTP proxy settings

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App version

6.602

Where did you get the app from?

F-Droid

Android version

LineageOS 20.0

Device model

XQ-AS72

Steps to reproduce

1. Manage a WiFi connection to add an HTTP proxy
2. Attempt to sync with K9
3. Message retrieval fails silently (no error message)
4. Attempt to compose an email
5. Receive the error `GaiException: android_getaddrinfo failed: EAI_NODATA (no address associated with hostname)

Expected behavior

K9 should follow system proxy settings and pass through the set HTTP proxy. Note that this is not related to #980 (as this is a system HTTP proxy, not a proxy set within the app itself), though there may be some overlap in the fix.

Actual behavior

Messages are not retrieved and emails cannot be sent.

Logs

k9mail-logs.txt

I've snipped a few things from the logs (namely a bunch of account information for my non-gmail accounts), so they may not line up perfectly.

Start with the attached k9mail-logs.txt and reproduce the failure using a WiFi HTTP proxy on LineageOS 20.0. Trace the sync and compose network paths to determine where the system proxy is bypassed; done means messages sync and email can be sent through the configured proxy without the reported address error.