fix(AppFramework): Log malformed protocol values and unify fallback behavior

[joshtrichards]

• Resolves: Nextcloud failing to setup: ActivityNotFoundException: No Activity found to handle Intent { act=android.intent.action.VIEW android#14306
• Resolves: Crash on Graphene OS after entering server address: No Activity found to handle Intent { act=android.intent.action.VIEW dat=HTTP: android#14474

While the above are configuration matters so they're now closed, we can handle it better here in server which is that this PR proposes.

Summary

This PR:

• Unifies existing fallback behavior regardless of method used to specify protocol (i.e. always returning http at a minimum).
• Adds logging whenever the fallback is used so the admin has a chance to fix whatever is causing it.

We currently:

• Fallback to a sane protocol value (http) when a malformed protocol value is received from a reverse proxy (via X-Forwarded-Proto).
• Silently fallback, which can make troubleshooting challenging.
• Trust the value of overwriteprotocol to be reasonable.
• Have no fallback behavior for malformed overwriteprotocol values (unlike X-Forwarded-Proto).

The fallback is not a default value. It's just means a reasonable protocol is returned. When the fallback is used, it's basically an error. If it wasn't existing behavior, we might consider throwing an exception here instead. This PR just adds similar fallback behavior for malformed overwriteprotocol values and also logs when the fallback is used.

Details:

• Sanity checks value overwriteprotocol (if any) and when malformed uses same fallback value (http) as we do for X-Forwarded-Proto
• Adds logging when the server protocol is malformed (regardless of source)
• Treats upper/mixed case values (i.e. HTTP / HTTPS) as valid for overwriteprotocol (just as it already does for X-Forwarded-Proto)
• Refactors code slightly (formatting, comments, eliminates unnecessary comparisons, minor structure change)

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)