Determine which IP address to use for sshfs

Summary:
There is currently an issue where the device doesn't send the correct IP address for sshfs when a VPN is present.

Instead of asking the device to find and send its address, we can store it from when the device link is created then reuse it.

Test Plan:
All unit tests pass.

In these different situations:

• without a VPN
• with a VPN running
• with a VPN started then stopped (the tun interface might still be there)

Try to remotely browse the device. It should work in all cases.

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, apol, nicolasfella, sredman

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D6730