Fix quality selection and latency stats using an asyncQuerySelector

[Shingyx]

I was looking into why stream qualities weren't displaying for me on the custom overlay, and found it was occurring because after clicking the ⚙️ settings button on the player, the menu doesn't appear until a split second later.

To fix this, I've updated the stream quality methods to use a new asyncQuerySelector instead of document.querySelector. I had to apply the same fix to the latency stats to ensure consistent stream quality behaviour, as they also interact with the settings menu.

This change to using async JS code means that awaiting for videoWebViewController.runJavaScript no longer guarantees that the previous task actually completed, which was causing issues with the current defaultToHighestQuality logic. I resolved this by updating this setting to call a new _setStreamQualityIndex(1) instead. This also has the benefit of not "flashing" the settings menu twice when using defaultToHighestQuality without the custom overlay.

Fixes #338 (and maybe #334?)

[tommyxchow]

LGTM! Fantastic work and explanation, thank you so much!

The PR check is failing but we can ignore it, looks to be due to some new lint rules in the newer Flutter versions. I'll fix them after merging.