WIP: macOS: Fix platform view threading issues

[knopp]

Fixes https://github.com/flutter/flutter/issues/96668

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [X] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [X] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [X] I listed at least one issue that this PR fixes in the description above.
• [X] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [X] I updated/added relevant documentation (doc comments with ///).
• [X] I signed the CLA.
• [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[zanderso]

From PR review triage: Adding the WIP label.

[knopp]

Closing in favor of https://github.com/flutter/engine/pull/37789.