Use -Werror in frameworks/native/opengl/tests

* Remove or comment out unused local variables.
* Remove unused function.
* Cast print argument type to fix format mismatch warning.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I81e80849ac36288932df235cb0d63bd870357d86
diff --git a/opengl/tests/gl_basic/Android.bp b/opengl/tests/gl_basic/Android.bp
index 881d8ce..5eed17e 100644
--- a/opengl/tests/gl_basic/Android.bp
+++ b/opengl/tests/gl_basic/Android.bp
@@ -3,6 +3,11 @@
 
     srcs: ["gl_basic.cpp"],
 
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+
     gtest: false,
 
     shared_libs: [