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/graphicsenv/Android.bp b/libs/graphicsenv/Android.bp
index f2686d5..9f99538 100644
--- a/libs/graphicsenv/Android.bp
+++ b/libs/graphicsenv/Android.bp
@@ -19,6 +19,8 @@
         "GraphicsEnv.cpp",
     ],
 
+    cflags: ["-Wall", "-Werror"],
+
     shared_libs: [
         "libnativeloader",
         "liblog",