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/Android.bp b/Android.bp
index 7ae27e8..d8a4f6a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -29,7 +29,11 @@
         "libdl",
         "libvndksupport",
     ],
-    cflags: ["-DQEMU_HARDWARE"],
+    cflags: [
+        "-DQEMU_HARDWARE",
+        "-Wall",
+        "-Werror",
+    ],
 
     header_libs: ["libhardware_headers"],
     export_header_lib_headers: ["libhardware_headers"],