hal: fix compiler warnings which are now treated as errors

With CLANG enabled and -Wall & -Werror being set in
LOCAL_CFLAGS, all warnings are treated as errors.

This commit fixes all the warnings most of which are
about unused variables and functions.

Change-Id: I32c6593fa0ad6a4fdca5dc8e6c76982a146a8bc6
diff --git a/hal/Android.mk b/hal/Android.mk
index 4c06288..c1e122d 100644
--- a/hal/Android.mk
+++ b/hal/Android.mk
@@ -285,7 +285,7 @@
     LOCAL_SHARED_LIBRARIES += libperipheral_client
 endif
 
-#LOCAL_CFLAGS += -Wall -Werror
+LOCAL_CFLAGS += -Wall -Werror
 
 LOCAL_COPY_HEADERS_TO   := mm-audio
 LOCAL_COPY_HEADERS      := audio_extn/audio_defs.h