CHERRY-PICK: Start shill after wifi setup is completed

This prevents shill from initiating WiFi connection before WiFi
driver/firmware is fully initialzied.

Bug: 25731975
TEST=Setup a Brillo device, verify the device still have WiFi
     connectivity after reboot

Change-Id: I641e559be30f5dbae3cb31e08ff7810cb7de20dc
(cherry picked from commit 9297f64f2a4a846904f2da0f7992575c098bbe3c)
diff --git a/shill.rc b/shill.rc
index ede4bf1..b05127b 100644
--- a/shill.rc
+++ b/shill.rc
@@ -6,3 +6,7 @@
     class late_start
     user root
     group system dbus inet
+    disabled
+
+on property:wifi-setup.complete=1
+    enable shill