Enable strict warnings and fix unused warnings

Test: builds without warnings
Change-Id: I0701a44cbd9a2adc405a1e253b57df7ee03a90b7
diff --git a/media/libeffects/preprocessing/Android.mk b/media/libeffects/preprocessing/Android.mk
index bd67aa1..60030ac 100644
--- a/media/libeffects/preprocessing/Android.mk
+++ b/media/libeffects/preprocessing/Android.mk
@@ -28,5 +28,6 @@
     -DWEBRTC_POSIX
 
 LOCAL_CFLAGS += -fvisibility=hidden
+LOCAL_CFLAGS += -Wall -Werror
 
 include $(BUILD_SHARED_LIBRARY)