Audio VTS: Test now use Android.mk

To validate the audio_policy_configuration.xml against a schema,
libxml2 is used.
Unfortunately, libxml2 uses Android.mk in oc-dev thus can not be used
from the tests which uses Android.bp.

It has been deemed safer to transform the tests to use Android.mk rather
than backporting the Android.bp patch from master.

Test: Compile
Bug: 35700978
Merged-In: I800e692a6ff4f64655007c33af7e34d879ee1132
Change-Id: Ie3f2069d60be369b4c37c27e9c1cf9372323a19c
Signed-off-by: Kevin Rocard <krocard@google.com>