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
diff --git a/core/main.mk b/core/main.mk
index c29731c..cfc39bb 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -360,6 +360,10 @@
   # Target is secure in user builds.
   ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=1
 
+  ifeq ($(user_variant),user)
+    ADDITIONAL_DEFAULT_PROPERTIES += ro.adb.secure=1
+  endif
+
   ifeq ($(user_variant),userdebug)
     # Pick up some extra useful tools
     tags_to_install += debug