Use -Werror in frameworks/native/cmds

* Remove unused variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Ifde10b651abbdb2ab1e236ebe47e25bf6c4c9b23
diff --git a/cmds/flatland/Android.mk b/cmds/flatland/Android.mk
index c295167..7aa111c 100644
--- a/cmds/flatland/Android.mk
+++ b/cmds/flatland/Android.mk
@@ -8,6 +8,8 @@
     Renderers.cpp   \
     Main.cpp        \
 
+LOCAL_CFLAGS := -Wall -Werror
+
 LOCAL_MODULE:= flatland
 
 LOCAL_MODULE_TAGS := tests