Downmix: Add AIDL placeholder implementation and vts test

Bug: 258124419
Test: atest VtsHalDownmixTargetTest

Change-Id: I6c202d474b738e0e0d242dc91cafa3892b9e9987
diff --git a/audio/aidl/default/Android.bp b/audio/aidl/default/Android.bp
index b9b8cd8..803880f 100644
--- a/audio/aidl/default/Android.bp
+++ b/audio/aidl/default/Android.bp
@@ -128,6 +128,7 @@
     shared_libs: [
         "libbassboostsw",
         "libbundleaidl",
+        "libdownmixsw",
         "libdynamicsprocessingsw",
         "libenvreverbsw",
         "libequalizersw",