Fix memory leaks in audioserver

The code for parsing the volumes configuration
was not freeing libxml2 resources. The code updated
to use the same parametrizations of std::unique_ptr
as the main de-serializer for the APM config.

Bug: 190683126
Test: see the repro steps in the bug
Change-Id: I660c5b22654a68c21bf14175520ee6b7f40d026e