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/init/Android.bp b/init/Android.bp
index 8c4f005..45ee754 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -20,7 +20,7 @@
     sanitize: {
         misc_undefined: ["signed-integer-overflow"],
     },
-    cppflags: [
+    cflags: [
         "-DLOG_UEVENTS=0",
         "-Wall",
         "-Wextra",