set ro.adb.secure=1 for user builds

In user builds, adb without authentication is always disabled. However,
it doesn't work with the defaults because authentication doesn't work
without having ro.adb.secure=1 set. It makes sense to offer builds with
no adb but not builds with adb present in an unusable state (especially
as the default).

Bug: 22220212
Bug: 21862859
Change-Id: Ic22300a70586fd57692ac59d6873ab16dde70919