audio policy: split implementation

Split audio policy service implementation to ease
further evolution:
AudioPolicyInterfaceImpl.cpp contains the implementation of
IAudioPolicyService interface
AudioPolicyClientImpl.cpp contains the implementation of the
AudioPolicyManager client interface
AudioPolicyService.cpp contains the rest of the code mostly
command threads implementation and pre processing management.

AudioPolicyService.cpp:

Change-Id: Ic48ba165532ef66e84a30894d01f750ed6619d02