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/libgrallocusage/Android.bp b/libgrallocusage/Android.bp
index cf03868..bcc0616 100644
--- a/libgrallocusage/Android.bp
+++ b/libgrallocusage/Android.bp
@@ -15,9 +15,12 @@
 cc_library_static {
     name: "libgrallocusage",
     vendor_available: true,
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
     cppflags: [
         "-Weverything",
-        "-Werror",
         "-Wno-c++98-compat-pedantic",
         // Hide errors in headers we include
         "-Wno-global-constructors",