Use -Werror in frameworks/native/libs

* Suppress unused-variable and user-defined warnings.
* Keep sign-compare warnings to fix later.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I6eee1532aa22c92ed19515ad5574734d1016d967
diff --git a/libs/hwc2on1adapter/Android.bp b/libs/hwc2on1adapter/Android.bp
index ec9cbf8..420a1f6 100644
--- a/libs/hwc2on1adapter/Android.bp
+++ b/libs/hwc2on1adapter/Android.bp
@@ -17,9 +17,13 @@
     vendor: true,
 
     clang: true,
+    cflags: [
+        "-Wall",
+        "-Werror",
+        "-Wno-user-defined-warnings",
+    ],
     cppflags: [
         "-Weverything",
-        "-Wall",
         "-Wunused",
         "-Wunreachable-code",