audio policy: add option to use new policy manager

Add build option USE_LEGACY_AUDIO_POLICY to use either new
audio policy manager in local AudioPolicyManager.cpp
or the legacy AudioPolicyManagerBase.cpp via the policy HAL.

New features will be implemented only by the new audio policy manager.
Platform customiization will be by config file or new policy HAL.

AudioPolicyClientImplLegacy.cpp copied from AudioPolicyClientImpl.cpp
AudioPolicyInterfaceImplLegacy.cpp copied from AudioPolicyInterfaceImpl.cpp

New implementations of AudioPolicyInterface and AudioPolicyClient talking directly to
AudioPolicyManager.

Change-Id: I7a320883a1de13de2c9295343e996addf2f3c154