Use -Werror in hardware/libhardware/modules

* Remove unused local variables.
* Remove or comment out unused static functions.
* Fix trivial bugs found by unused value warnings.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I99389b883c89551850180d25241a35a40bb77b26
diff --git a/modules/sensors/Android.mk b/modules/sensors/Android.mk
index ca277e6..5ef6a97 100644
--- a/modules/sensors/Android.mk
+++ b/modules/sensors/Android.mk
@@ -25,7 +25,7 @@
 LOCAL_MODULE_RELATIVE_PATH := hw
 LOCAL_PROPRIETARY_MODULE := true
 
-LOCAL_CFLAGS := -DLOG_TAG=\"MultiHal\"
+LOCAL_CFLAGS := -Wall -Werror -DLOG_TAG=\"MultiHal\"
 
 LOCAL_SRC_FILES := \
     multihal.cpp \