Downmix: Add AIDL placeholder implementation and vts test

Bug: 258124419
Test: atest VtsHalDownmixTargetTest

Change-Id: I6c202d474b738e0e0d242dc91cafa3892b9e9987
diff --git a/audio/aidl/default/audio_effects_config.xml b/audio/aidl/default/audio_effects_config.xml
index d22c349..c08f913 100644
--- a/audio/aidl/default/audio_effects_config.xml
+++ b/audio/aidl/default/audio_effects_config.xml
@@ -30,6 +30,7 @@
     <libraries>
         <library name="bassboostsw" path="libbassboostsw.so"/>
         <library name="bundle" path="libbundleaidl.so"/>
+        <library name="downmixsw" path="libdownmixsw.so"/>
         <library name="dynamics_processingsw" path="libdynamicsprocessingsw.so"/>
         <library name="equalizersw" path="libequalizersw.so"/>
         <library name="haptic_generatorsw" path="libhapticgeneratorsw.so"/>
@@ -65,6 +66,7 @@
             <libsw library="bassboostsw" uuid="fa8181f2-588b-11ed-9b6a-0242ac120002"/>
             <libsw library="bundle" uuid="8631f300-72e2-11df-b57e-0002a5d5c51b"/>
         </effectProxy>
+        <effect name="downmix" library="downmixsw" uuid="fa8187ba-588b-11ed-9b6a-0242ac120002"/>
         <effect name="dynamics_processing" library="dynamics_processingsw" uuid="fa818d78-588b-11ed-9b6a-0242ac120002"/>
         <effect name="haptic_generator" library="haptic_generatorsw" uuid="fa819110-588b-11ed-9b6a-0242ac120002"/>
         <effect name="loudness_enhancer" library="loudness_enhancersw" uuid="fa819610-588b-11ed-9b6a-0242ac120002"/>