vndservicemanager: load vendor service_contexts.

Also, don't crash when /dev/vndbinder is not
accessible.

Bug: 36052864
Test: loads vendor service contexts
Test: doesn't crash without /dev/vndbinder, works with it
Change-Id: Ida9dd94791206e14246348a8b8e66be09ed8fca7
diff --git a/cmds/servicemanager/Android.bp b/cmds/servicemanager/Android.bp
index 5431233..68d39db 100644
--- a/cmds/servicemanager/Android.bp
+++ b/cmds/servicemanager/Android.bp
@@ -43,6 +43,9 @@
         "service_manager.c",
         "binder.c",
     ],
+    cflags: [
+        "-DVENDORSERVICEMANAGER=1",
+    ],
     shared_libs: ["libcutils", "libselinux"],
     init_rc: ["vndservicemanager.rc"],
 }