[tvOS][widevine] Use different pthread_setname_np() on Apple platforms

[abhijeetk]

These changes ensure WidevineTimer follows the same portable pthread
patterns established in commits 9d29311 and 42a3596, and
correctly handles platform-specific pthread_setname_np() semantics on
Apple platforms.

Bug: 432507888
Bug: 438955200
Bug: 471365322

[github-actions]

🤖 Gemini Suggested Commit Message

---

starboard: Adapt pthread_setname_np for Apple

This change updates WidevineTimer to use std::optional for its thread
handle, aligning with existing portable pthread patterns. It also
introduces platform-specific handling for pthread_setname_np on Apple
platforms, where the function signature differs. This ensures correct
thread naming and overall portability for Widevine's timer mechanism.

Bug: 432507888
Bug: 438955200

---

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[gemini-code-assist]

Code Review

This pull request correctly addresses the platform-specific signature of pthread_setname_np() on Apple platforms by using preprocessor directives. The related refactoring of the thread handle thread_ from a raw pthread_t to std::optional<pthread_t> is a welcome improvement, making the code more modern, safer, and more expressive. The logic for thread creation and destruction appears sound. I have one minor suggestion to make the code slightly more idiomatic when using std::optional.

[abhijeetk]

cc : @Gyuyoung @jkim-julie @rakuco