[iOS] Improve new playlist UI responsiveness

[kylehickinson]

This change adds async versions to many synchronous AVKit operations that were being called in Playlist such as loading the duration of the video, fetching the download state, and setting the duration in PlayerModel once per load rather than have it computed.

Resolves brave/brave-browser#42501

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

In addition to STR in the issue, also test these things on both old and new playlist:

• Downloading a video for offline correctly fetches the duration and displays it correctly in the list

[brave-builds]

Released in v1.75.50

[Uni-verse]

Verified on iPhone 12 running iOS 18.0 using version 1.75 (52)

• Ensured following steps in Playlist UI can be unresponsive and hang brave-browser#42501 (comment) does not cause any freezing / unresponsiveness.
• Ensured videos have the correct duration in the list when saving offline for old and new Playlist UIs following test plan in [iOS] Improve new playlist UI responsiveness #26726 (comment)
• Spot checked other playlist functions to make sure nothing has regressed.

old UI	new UI	saving offline