Error fix from enabling Clang thread-safety checks.

Parent bug: b/353324127. Enabling `thread-safety-reference-return`
to allow developers to utilize Clang thread safety guardrails
(GUARDED_BY, REQUIRES, EXCLUDES).

Error:
frameworks/av/services/audioflinger/AudioFlinger.h:337:71: error:
returning variable 'mAudioHwDevs' by reference requires holding
mutex 'hardwareMutex()' [-Werror,-Wthread-safety-reference-return]

Fix: return by value, hold mutex from parent, or remove GUARDED_BY
attribute.

This fix removes the GUARDED_BY attribute in order to avoid changing
functionality. If this is not desired, please respond with concerns
or preferred fix.

Bug: 354744310
Test: make
Change-Id: Ie666fe5a0e915dc9b558e0daa5bb9f927f392567