audio_extn: Allow skipping compilation of audio HAL extensions

In case a device uses a prebuilt HAL it is rather likely
that the extensions must match it.

Change-Id: I0cb05030c9c7619513810362960a2e2a3306b3aa
diff --git a/hal/audio_extn/Android.mk b/hal/audio_extn/Android.mk
index fc07feb..a465d65 100644
--- a/hal/audio_extn/Android.mk
+++ b/hal/audio_extn/Android.mk
@@ -1,3 +1,5 @@
+ifneq ($(strip $(TARGET_PROVIDES_AUDIO_EXTNS)),true)
+
 #--------------------------------------------
 #          Build SND_MONITOR LIB
 #--------------------------------------------
@@ -961,3 +963,5 @@
 LOCAL_SANITIZE := integer_overflow
 endif
 include $(BUILD_SHARED_LIBRARY)
+
+endif