Use -Werror in system/core

* Move -Wall -Werror from cppflags to cflags.
* Fix/suppress warning on unused variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I1e05e96a1d0bcb2ccef1ce456504b3af57167cc5
diff --git a/libutils/tests/Android.bp b/libutils/tests/Android.bp
index a891fca..a3e7ffe 100644
--- a/libutils/tests/Android.bp
+++ b/libutils/tests/Android.bp
@@ -77,6 +77,7 @@
     host_supported: true,
     relative_install_path: "libutils_tests",
     srcs: ["Singleton_test1.cpp"],
+    cflags: ["-Wall", "-Werror"],
 }
 
 cc_test_library {
@@ -84,5 +85,6 @@
     host_supported: true,
     relative_install_path: "libutils_tests",
     srcs: ["Singleton_test2.cpp"],
+    cflags: ["-Wall", "-Werror"],
     shared_libs: ["libutils_tests_singleton1"],
 }