Use -Werror in hardware/libhardware

* Remove unused local variables and function.
* Fix unused return value warning.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I269eca76cda2222c7a0176e85e92df0bb0924a0e
diff --git a/tests/nusensors/Android.bp b/tests/nusensors/Android.bp
index e923468..8b267db 100644
--- a/tests/nusensors/Android.bp
+++ b/tests/nusensors/Android.bp
@@ -3,6 +3,8 @@
 
     srcs: ["nusensors.cpp"],
 
+    cflags: ["-Wall", "-Werror"],
+
     shared_libs: [
         "libcutils",
         "libhardware",