AudioFlinger: Add ThreadBase and PlaybackThread interfaces

ThreadBase::standby() check renamed ThreadBase::inStandby() to avoid future
confusion with an active method.

Test: atest audiorecord_tests audiotrack_tests audiorouting_tests trackplayerbase_tests audiosystem_tests
Test: atest AudioTrackTest AudioRecordTest
Test: YouTube Camera
Bug: 288339104
Bug: 289233517
Change-Id: Ied45a5d762b5a53c9d5dcd88d26efaa8b058836d
diff --git a/services/audioflinger/datapath/VolumeInterface.h b/services/audioflinger/datapath/VolumeInterface.h
index 4635e07..1564fe1 100644
--- a/services/audioflinger/datapath/VolumeInterface.h
+++ b/services/audioflinger/datapath/VolumeInterface.h
@@ -23,6 +23,7 @@
 class VolumeInterface : public virtual RefBase {
 public:
     virtual void setMasterVolume(float value) = 0;
+    virtual void setMasterBalance(float balance) = 0;
     virtual void setMasterMute(bool muted) = 0;
     virtual void setStreamVolume(audio_stream_type_t stream, float value) = 0;
     virtual void setStreamMute(audio_stream_type_t stream, bool muted) = 0;