audio policy: enable use of new AudioPolicyManager

Do not define USE_LEGACY_AUDIO_POLICY by default.
By default, all devices will now use the new audio policy implementation
in AudioPolicyManager.cpp

To continue using the legacy audio policy implemented by
AudioPolicyManagerBase in libhardware_legacy a device should
define USE_LEGACY_AUDIO_POLICY in its makefile.

Change-Id: I7b8cac3cbb24b11105ba6233501c7212f5985727