Do not lower measured bandwidth with Network Information API

[PascalThuet]

Description

Fixes #1604.

When useNetworkInformationApi is enabled, VHS currently uses navigator.connection.downlink as the public bandwidth value whenever Network Information API data is present, except for the existing >= 10 Mbps special case. That can let a lower generic browser-level estimate override a higher player-specific bandwidth estimate after VHS has downloaded media segments.

This change keeps the Network Information API behavior before media segment stats are available, and still allows it to raise the estimate afterward. Once the media segment loader has a player-specific estimate, however, Network Information API no longer lowers that value.

Specific Changes proposed

• Track whether the main segment loader has a bandwidth estimate from media segment stats.
• Preserve the existing Network Information API behavior before media segment stats are available.
• After media segment stats are available, use the larger of the player estimate and the Network Information API estimate.
• Add unit coverage for both post-segment-stats cases:
  • Network Information API lower than the player estimate.
  • Network Information API higher than the player estimate.
• Update the useNetworkInformationApi README entry to describe the post-segment-stats behavior.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

Validation:

• npm run lint
• CI_TEST_TYPE=unit npm run build-test
• CI_TEST_TYPE=unit npx karma start scripts/karma.conf.js --single-run --browsers ChromeHeadless