audio policy: add call assistant volume stream

Routing strategy call assistant was added without
associated volume stream and wass reusing AUDIO_STREAM_PATCH.
This is violating the rule that two strategies cannot
share the same volume stream.

Bug:148588565
Test: play music and check volume
Change-Id: Ib3fb2a1b7b5f3118bf16a3cc9d8e578fa0431645