Downmix: Add AIDL placeholder implementation and vts test

Bug: 258124419
Test: atest VtsHalDownmixTargetTest

Change-Id: I6c202d474b738e0e0d242dc91cafa3892b9e9987
diff --git a/audio/aidl/TEST_MAPPING b/audio/aidl/TEST_MAPPING
index b4607f9..dfd82c3 100644
--- a/audio/aidl/TEST_MAPPING
+++ b/audio/aidl/TEST_MAPPING
@@ -10,6 +10,9 @@
       "name": "VtsHalAudioEffectTargetTest"
     },
     {
+      "name": "VtsHalDownmixTargetTest"
+    },
+    {
       "name": "VtsHalEqualizerTargetTest"
     },
     {