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 446eece..baa28f2 100644
--- a/hal/audio_extn/Android.mk
+++ b/hal/audio_extn/Android.mk
@@ -1,3 +1,5 @@
+ifneq ($(strip $(TARGET_PROVIDES_AUDIO_EXTNS)),true)
+
 #AudioHal-primaryHal-Hal path
 ifneq ($(BOARD_OPENSOURCE_DIR), )
   PRIMARY_HAL_PATH := $(BOARD_OPENSOURCE_DIR)/audio-hal/primary-hal/hal
@@ -1274,3 +1276,4 @@
 
 include $(BUILD_SHARED_LIBRARY)
 endif
+endif