Start apmanager after WiFi setup is completed

This avoids a race condition where weaved tries to start an AP service
before WiFi driver is initialized.

Bug: 25341872
TEST=Device setup on both Brillo boards

Change-Id: Iadaa7a6b18dab5adb21ba02d12d7704dabb40332
diff --git a/apmanager.rc b/apmanager.rc
index 3e1fed0..3308990 100644
--- a/apmanager.rc
+++ b/apmanager.rc
@@ -7,3 +7,7 @@
     class main
     user system
     group system dbus net_admin net_raw
+    disabled
+
+on property:wifi-setup.complete=1
+    enable apmanager