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 5cba5042)