Do not add 'adb' to persist.sys.usb.config if already there

The 'adb' configuration might have already been added by a
device-specific Makefile, so add it only if it's not already there.

Change-Id: I6f96645f44c96f6f827cc8c842a8b769f92be13a

tools/post_process_props.py

@@ -37,11 +37,12 @@ def mangle_default_prop(prop):
   # (this is for userdebug builds)
   if prop.get("ro.debuggable") == "1":
     val = prop.get("persist.sys.usb.config")
-    if val == "":
-      val = "adb"
-    else:
-      val = val + ",adb"
-    prop.put("persist.sys.usb.config", val)
+    if "adb" not in val:
+      if val == "":
+        val = "adb"
+      else:
+        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.