UI: Only cache multitrack config URL startup argument

[palana]

Description

Allow usage of different multitrack config URLs between different stream attempts without restarting the program

Motivation and Context

This is an artifact from when development on multitrack video started, before services could supply their config URLs through services.json

This currently works when switching between Twitch and Amazon IVS due to some infrastructure being shared, but won't work if switching between Twitch/Amazon IVS and a completely different service

How Has This Been Tested?

Switched between Twitch and Amazon IVS and verified that they use their own service URLs:

13:27:18.508: Go live URL: https://ingest.contribute.live-video.net/api/v3/GetClientConfiguration
[…]
13:27:38.260: Go live URL: https://ingest.twitch.tv/api/v3/GetClientConfiguration

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[derrod]

I don't think you need to explicitly call .has_value() in an if expression but it's fine.