Audio VTS: Create utility lib

Move code that can be reuse between audio tests of the HIDL interface in
an utility library.

Test: run vts tests
Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Bug: 35700978

Change-Id: Ia930ad2b0a3ca31628b2cb767b265d54b210756b
Signed-off-by: Kevin Rocard <krocard@google.com>