Make audio HAL selection configurable for Trout downstreams

Test: build and flash the downstream target
Bug: 170770260
Change-Id: Ic266c325170316358cdd754d7e53c66f15dafe7e
diff --git a/aosp_trout_arm64.mk b/aosp_trout_arm64.mk
index b1446e7..ef1fa10 100644
--- a/aosp_trout_arm64.mk
+++ b/aosp_trout_arm64.mk
@@ -17,7 +17,7 @@
 $(call inherit-product, device/google/cuttlefish/vsoc_arm64/auto/aosp_cf.mk)
 
 # Audio HAL
-TARGET_USES_CUTTLEFISH_AUDIO := false
+TARGET_USES_CUTTLEFISH_AUDIO ?= false
 
 # Audio Control HAL
 # TODO (chenhaosjtuacm, egranata): move them to kernel command line