audiopolicy: update APM to use custom audio policy configuration

Update AudioPolicyManager to use custom audio_policy_configuration.xml
if available.
This is to avoid keeping qti value added customization in default policy
configuration, since with treble design vts expects default vendor
policy configuration to be supported even with aosp system image but that
aosp system image would not have qti customization support and hence might
result in failures.

Change-Id: I3161ff4fa41d37d0b761202f0b3a490d9c7e418e