Add audio stream and usage for virtual assistant

New stream type: AUDIO_STREAM_ASSISTANT
This is intended to be used by a virtual assistant like
Google Assistant, Bixby, etc.

The audio stream has own volume alias and the volume
does not change by volume changes of other streams.

Bug: 123745215
Test: make
Change-Id: I3696d8fba32070954cc6a330574af8507fea7a74
Merged-In: I3696d8fba32070954cc6a330574af8507fea7a74
diff --git a/audio/effect/6.0/xml/api/current.txt b/audio/effect/6.0/xml/api/current.txt
index 2021639..2dfcb9b 100644
--- a/audio/effect/6.0/xml/api/current.txt
+++ b/audio/effect/6.0/xml/api/current.txt
@@ -82,6 +82,7 @@
   public enum StreamOutputType {
     method public String getRawName();
     enum_constant public static final audio.effects.V6_0.StreamOutputType alarm;
+    enum_constant public static final audio.effects.V6_0.StreamOutputType assistant;
     enum_constant public static final audio.effects.V6_0.StreamOutputType bluetooth_sco;
     enum_constant public static final audio.effects.V6_0.StreamOutputType dtmf;
     enum_constant public static final audio.effects.V6_0.StreamOutputType enforced_audible;