Ignore ro.adb.secure in user builds.

Require authorization by default, and remove the ability to override
that in user builds. (userdebug and eng are still free to do whatever
they want.)

Bug: http://b/21862859
Change-Id: Ibf8af375be5bf1141c1ad481eee7a59fb10a7adb
(cherry picked from commit 5cba504215ea91187cc36ec7aec5dce1b0f4b0fe)
diff --git a/adb/adb_auth.h b/adb/adb_auth.h
index 1e1978d..a13604a 100644
--- a/adb/adb_auth.h
+++ b/adb/adb_auth.h
@@ -19,7 +19,7 @@
 
 #include "adb.h"
 
-extern int auth_enabled;
+extern bool auth_required;
 
 int adb_auth_keygen(const char* filename);
 void adb_auth_verified(atransport *t);