Audio: Add XML schema for audio policy configuration

The schema describes the XML format for audio policy
configuration.
It will be enforced in a follow up vts test.

Bug: 35700978
Test: schema is parsed with xmllint with no errors
Test: xmllint  --xinclude --noout --schema audio_policy_configuration.xsd  out/target/product/sailfish/vendor/etc/audio_policy_configuration.xml

Change-Id: Ic6830446b54b3e02e4bca9b0a392523a8fb359e1
Signed-off-by: Kevin Rocard <krocard@google.com>