Use -Werror in frameworks/native/libs/gui and ui

* Remove unused variable and function.
* Suppress warning of unused return results.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: If6f445be1e7677a37bf780844a79b84d6d7e1c1e
diff --git a/libs/gui/tests/Android.bp b/libs/gui/tests/Android.bp
index fa87f29..908959c 100644
--- a/libs/gui/tests/Android.bp
+++ b/libs/gui/tests/Android.bp
@@ -7,6 +7,10 @@
     test_suites: ["device-tests"],
 
     clang: true,
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
 
     srcs: [
         "BufferItemConsumer_test.cpp",