Fix warnings and enable -Werror

Fix unused-variable and empty for loop body warnings, disable
enum-conversion warnings, and enable -Wall -Werror

Test: builds
Change-Id: I7f349b29b1c64f4b4c3be9cae8b543fedf133d4d
diff --git a/Android.bp b/Android.bp
index 14291fc..173db03 100644
--- a/Android.bp
+++ b/Android.bp
@@ -104,6 +104,9 @@
     ],
 
     cflags: [
+        "-Wall",
+        "-Werror",
+        "-Wno-enum-conversion",
         "-Depl_LINUX",
         "-Wno-unused-parameter",
     ],