Turn off adb on user builds.

Tungsten is the first device we have done since post_process_props.py
and UsbDeviceManager were added that by default has no services running
on usb. Everything else has mtp.  This makes it so we always populate
the property with something.

Change-Id: Ic24f52462ed8866e87499c715dd21937e5b1128d
diff --git a/tools/post_process_props.py b/tools/post_process_props.py
index 01f9128..9d69736 100755
--- a/tools/post_process_props.py
+++ b/tools/post_process_props.py
@@ -21,7 +21,6 @@
 def mangle_build_prop(prop):
 # Put the modifications that you need to make into the /system/build.prop into this
 # function. The prop object has get(name) and put(name,value) methods.
 def mangle_default_prop(prop):
@@ -34,7 +33,11 @@
       val = val + ",adb"
     prop.put("persist.sys.usb.config", val)
+  # UsbDeviceManager expects a value here.  If it doesn't get it, it will
+  # default to "adb". That might not the right policy there, but it's better
+  # to be explicit.
+  if not prop.get("persist.sys.usb.config"):
+    prop.put("persist.sys.usb.config", "none");
 class PropFile:
   def __init__(self, lines):