hal: Fix warnings in audio HAL and policy

Enable Wall and Werror and fix warnings in
HAL and policy manager

CRs-Fixed: 962049

Change-Id: Ie93c8f933943875ac68e8318d777602ed864f531
diff --git a/hal/Android.mk b/hal/Android.mk
index ea54bd5..229b3f4 100644
--- a/hal/Android.mk
+++ b/hal/Android.mk
@@ -282,6 +282,8 @@
     LOCAL_SHARED_LIBRARIES += libperipheral_client
 endif
 
+LOCAL_CFLAGS += -Wall -Werror
+
 LOCAL_COPY_HEADERS_TO   := mm-audio
 LOCAL_COPY_HEADERS      := audio_extn/audio_defs.h