AudioFlinger: Move NBLog thread_local to no linkage.

Access the thread_local by getter/setter only.
This prevents a memory leak with the clang compiler.

Test: adb shell dumpsys media.audio_flinger --unreachable
Bug: 284657986
Merged-In: Ic7128eea2aef47289c6e38ebca6fb5292d919410
Change-Id: Ic7128eea2aef47289c6e38ebca6fb5292d919410
(cherry picked from commit 5a5b514a)