audio VTS: Consider only attached devices for stream tests

V6 and V7 use the audio policy configuration file for generating
the set of test profiles. The generator must only take into
account profiles of mix ports that are routed to device ports of
attached devices. Otherwise the HAL can rightfully refuse
to create a stream for the profile when the device isn't attached.

Bug: 205808571
Test: atest HalAudioV6_0GeneratorTest
Test: atest HalAudioV7_0GeneratorTest
Test: atest VtsHalAudioV6_0TargetTest
Test: atest VtsHalAudioV7_0TargetTest
Change-Id: I798f978c1c0bc32831c20e4b08338b462d8eea16
Merged-In: I798f978c1c0bc32831c20e4b08338b462d8eea16