audio: Remove dependency on libxml2 from the default implementation

The default implementation switched to use the "enums"
module generated from the audio_policy_configuration.xsd schema.

This drops the dependency on libxml2 library and reduces
PSS and private dirty memory usage.

Bug: 180413957
Test: atest VtsHalAudioV7_0TargetTest
Change-Id: If7c110cd2bd2698923957b39906a0896d18065dd