dexopt: Only dexopt boot and system server for non-user builds

As stated in https://android.googlesource.com/platform/build/+/418258c
this is required in O-MR1 since sepolicy now prevents system server from
loading anything from /data making our system extremely slow and janky.

Change-Id: I49a97a75ed050f1f44e7c4c19332f6c7b76dd5db