Downmix: Add AIDL placeholder implementation and vts test

Bug: 258124419
Test: atest VtsHalDownmixTargetTest

Change-Id: I6c202d474b738e0e0d242dc91cafa3892b9e9987
diff --git a/audio/aidl/vts/Android.bp b/audio/aidl/vts/Android.bp
index 4414e26..30f988d 100644
--- a/audio/aidl/vts/Android.bp
+++ b/audio/aidl/vts/Android.bp
@@ -73,6 +73,12 @@
 }
 
 cc_test {
+    name: "VtsHalDownmixTargetTest",
+    defaults: ["VtsHalAudioTargetTestDefaults"],
+    srcs: ["VtsHalDownmixTargetTest.cpp"],
+}
+
+cc_test {
     name: "VtsHalEqualizerTargetTest",
     defaults: ["VtsHalAudioTargetTestDefaults"],
     srcs: ["VtsHalEqualizerTargetTest.cpp"],