Added get_audio_flinger_for_fuzzer in AudioSystem.cpp

get_audio_flinger_for_fuzzer() is added in AudioSystem
to disable the creation of thread pools. This function is used
by the audioflinger_aidl_fuzzer and auidopolicy_aidl_fuzzer
fuzzes to avoid the abort() call from fuzzerService()
due to thread pool creation.

Test: make libaudioclient
Bug: 202897137

Merged-In: I13c75cbd89b09089851304f7177ef8f0bf9d39f9
Change-Id: I13c75cbd89b09089851304f7177ef8f0bf9d39f9