AudioSystem: fix cross deadlock

Do not hold gLockAPS when calling
AudioPolicyService::registerClient() in get_audio_policy_service().
registerClient() will need to acquire the AudioPolicyService mutex and
if at the same time a method called from AudioPolicyService
(with mutex held) calls back into AudioSystem and get_audio_policy_service()
a cross deadlock occurs.

Same preventive fix for get_audio_flinger().

Use a separate mutex for notification client list in AudioPolicyService.
This prevents deadlocking if registerClient() is called as a consequence of
AudioFlinger calling back into AudioPolicyManager while executing a method
with AudioPolicyService locked

Bug: 18403952.
Bug: 18450065.
Change-Id: Ia832e41aede8bc6c843fc615508fbdd74e0863b5