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/modules/sensors/Android.bp b/modules/sensors/Android.bp
index 1d15065..64cf2a9 100644
--- a/modules/sensors/Android.bp
+++ b/modules/sensors/Android.bp
@@ -12,4 +12,5 @@
         "libdl"
     ],
     export_include_dirs: ["."],
+    cflags: ["-Wall", "-Werror"],
 }